一个工程师的自我修养

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1.python解题Leetcode? + details

2.java解题src/Solution? + details

2.C++解题CPP/Solution? + details

4.特别部分random

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面试过

• EggTopFloor

  • Tencent - Machine learning Engineer

• HuffmanEncoding

  • Didichuxing - Statistical Analyst

• 最大公共字符串长度

  • 字节跳动

• 素数伴侣

  • HuaWei - Machine learning Engineer
  • 核心在于匈牙利算法

• Lru_Cache

  • 360 - Deep learning Engineer

• 消消乐

  • Tencent - Deep learning Engineer

• 花匠摆花

  • Tencent - Deep learning Engineer

  • source from:http://codeforces.com/contest/474/problem/D

• 意图相似度

  • 阿里 - Algorithm Engineer

• 数组分组问题

  • 携程 - Deep learning Engineer

• 连乘质数和

  • 快手2020校招

• binarySearchTree

  • 搜索二叉树

• 文本编辑距离

• 01背包

• 完全背包

• 多重背包

  • 三个都是一个套路，以01背包进行拓展

• 约瑟夫环问题

• 按递增间隔删数，直到最后只剩下一个数

• SortAlgorithm

  • Almost everytime such as Alibaba、JD、Didichuxing...

• GapCount

  • Hp(Hewlett-Packard) - Model Engineer

• 无序数组排序（时间复杂度为O(n)）

  • 蚂蚁金服

• 计数排序

  • 阿里-推荐算法工程师

• 插入排序

• 希尔排序

• 归并排序

• 堆排序

• 切钢条

• 切钢条

  • 阿里-数据智能技术架构师

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数组

• two-sum

  • source from:https://leetcode-cn.com/problems/two-sum/

• container-with-most-water

  • source from:https://leetcode-cn.com/problems/container-with-most-water/

• 3Sum

  • 左右双指针,亮点在于定义了很多提前跳出条件，在相同值跳过的条件判断中用nums[i+1]还是nums[i-1]尤为灵活

  • source from:https://leetcode-cn.com/problems/3sum/

• 3Sum Closest

  • 这题和上面一题很类似，只是最好不要在多生产变量了，直接拿num[first]+num[last]+num[i]去和target比，而不要考虑差值是否大于0，很容易绕进去

  • source from:https://leetcode-cn.com/problems/3sum-closest/

• 4Sum

  • source from:https://leetcode-cn.com/problems/4sum/

• Remove Duplicates from Sorted Array

  • source from:https://leetcode-cn.com/problems/remove-duplicates-from-sorted-array/

• Remove Element

  • source from:https://leetcode-cn.com/problems/remove-element/

• search-in-rotated-sorted-array

  • 螺旋数组二分

  • source from:https://leetcode-cn.com/problems/search-in-rotated-sorted-array/

• find-first-and-last-position-of-element-in-sorted-array

  • 这个考的是二分法中的while跳出条件和left、right变化逻辑，本题是找出左右边界

  • source from:https://leetcode-cn.com/problems/find-first-and-last-position-of-element-in-sorted-array/

• Search Insert Position

  • source from:https://leetcode-cn.com/problems/search-insert-position/

• combination-sum

  • 这题和上面这题很像，递归在for循环里面的回溯算法，把所有情况跑一遍。画重点，求解过程中存着result 是List<List<Integer>>的，而tmp_list是list的,需要把tmp_list再裹一层new ArrayList<>(tmp_list)再进行add

  • source from:https://leetcode-cn.com/problems/combination-sum/

• trapping-rain-water

  • 动态规划和栈的两种方法，很值得看一下

  • source from:https://leetcode-cn.com/problems/trapping-rain-water/

• rotate-image

  • source from:https://leetcode-cn.com/problems/rotate-image/

• jump-game

  • 记录上次每次可跳的最远距离

  • source from:https://leetcode-cn.com/problems/jump-game/

• MergeIntervals

  • source from:https://leetcode-cn.com/problems/merge-intervals/

• SpiralMatrixII

  • 旋转矩阵题型，用了余数判断位置，比较亮点

  • source from:https://leetcode-cn.com/problems/spiral-matrix-ii/

• unique-paths-ii

  • source from:https://leetcode-cn.com/problems/unique-paths-ii/

• sort-colors

  • source from:https://leetcode-cn.com/problems/sort-colors/

• Subsets

  • 这题利用了python的语言机制，很有意思

  • source from::https://leetcode.com/problems/subsets/

• word-search

  • source from:https://leetcode-cn.com/problems/word-search/

• largest-rectangle-in-histogram

  • source from:https://leetcode-cn.com/problems/largest-rectangle-in-histogram/

• merge-sorted-array

• merge-sorted-array

  • source from:https://leetcode-cn.com/problems/merge-sorted-array/

• palindrome-partitioning-ii

  • 状态二维dp+值一维dp
  • 半三角dp修正
  • source from:https://leetcode-cn.com/problems/palindrome-partitioning-ii/

• majority-element

  • source from:https://leetcode-cn.com/problems/majority-element/

• product-of-array-except-self

  • 两侧分别扫描一遍

  • source from:https://leetcode-cn.com/problems/product-of-array-except-self/

• move-zeroes

  • source from:https://leetcode-cn.com/problems/move-zeroes/

• find-the-duplicate-number

  • 抽屉原理+环+快慢指针

  • source from:https://leetcode-cn.com/problems/find-the-duplicate-number/

• find-all-duplicates-in-an-array

  • 和上面一样，核心是每次对abs(nums[idx])-1的处理获得nums的index很有意思

  • source from:https://leetcode-cn.com/problems/find-all-duplicates-in-an-array/

• find-all-numbers-disappeared-in-an-array

  • nums[abs(nums[idx]) - 1] = -abs(nums[abs(nums[idx]) - 1]),两个abs保证了所有存在的idx都被置负了

  • source from:https://leetcode-cn.com/problems/find-all-numbers-disappeared-in-an-array/

• max-consecutive-ones

  • source from:https://leetcode-cn.com/problems/max-consecutive-ones/

• teemo-attacking

  • source from:https://leetcode-cn.com/problems/teemo-attacking/

• SubarraySumEqualsK

  • source from:https://leetcode-cn.com/problems/subarray-sum-equals-k/

• shortest-unsorted-continuous-subarray

  • source from:https://leetcode-cn.com/problems/shortest-unsorted-continuous-subarray/

• can-place-flowers

  • source from:https://leetcode-cn.com/problems/can-place-flowers/

• valid-triangle-number

• valid-triangle-number

  • source from:https://leetcode-cn.com/problems/valid-triangle-number/

• task-scheduler

  • 理解一下，出现次数最多的那个字母（出现次数-1）x时间窗口及为最小次数，剩余要考虑的是都为最多次数的字母数

  • source from:https://leetcode-cn.com/problems/task-scheduler/

• maximum-average-subarray-i

  • source from:https://leetcode-cn.com/problems/maximum-average-subarray-i/

• maximum-swap

• maximum-swap

  • source from:https://leetcode-cn.com/problems/maximum-swap/

• find-pivot-index

• find-pivot-index

  • source from:https://leetcode-cn.com/problems/find-pivot-index/

• largest-number-at-least-twice-of-others

  • source from:https://leetcode-cn.com/problems/largest-number-at-least-twice-of-others/

• transpose-matrix

  • source from:https://leetcode-cn.com/problems/transpose-matrix/

• AdvantageShuffle

  • 这道题的难点在于耗时，贪心算法即可，田忌赛马，可以看下

  • source from:https://leetcode-cn.com/problems/advantage-shuffle/

• sort-array-by-parity-ii

  • source from:https://leetcode-cn.com/problems/sort-array-by-parity-ii/

• subarray-sums-divisible-by-k

• subarray-sums-divisible-by-k

  • 前缀和技巧题，主要是要知道a-b能被k整除，那么有a%k==b%k
  • source from:https://leetcode-cn.com/problems/subarray-sums-divisible-by-k/

• squares-of-a-sorted-array

  • source from:https://leetcode-cn.com/problems/squares-of-a-sorted-array/

• add-to-array-form-of-integer

  • source from:https://leetcode-cn.com/problems/add-to-array-form-of-integer/

• binary-prefix-divisible-by-5

  • source from:https://leetcode-cn.com/problems/binary-prefix-divisible-by-5/

• HeightChecker

  • source from:https://leetcode-cn.com/problems/height-checker/

• find-n-unique-integers-sum-up-to-zero

• find-n-unique-integers-sum-up-to-zero

  • source from:https://leetcode-cn.com/problems/find-n-unique-integers-sum-up-to-zero/

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动态规划

• regular-expression-matching

  • 分N种情况讨论状态转移矩阵。复杂：1、转移矩阵，2、初始化逻辑
  • source from:https://leetcode-cn.com/problems/regular-expression-matching/

• jump-game-ii

  • source from:https://leetcode-cn.com/problems/jump-game-ii/

• longest-palindromic-substring

  • 中心展开，从中间值往两边展开，判断展开终止点的长度

  • source from:https://leetcode-cn.com/problems/longest-palindromic-substring/

• maximum-subarray

  • 简单递归

  • source from:https://leetcode-cn.com/problems/maximum-subarray/

• unique-paths

  • source from:https://leetcode-cn.com/problems/unique-paths/

• unique-paths-ii

• unique-paths-ii

  • 最优路径下的dp

  • source from:https://leetcode-cn.com/problems/unique-paths-ii/

• minimum-path-sum

  • 非常基础简单的动态规划问题，适合理解动态规范的想法

  • source from:https://leetcode-cn.com/problems/minimum-path-sum/

• maximal-rectangle

  • source from:https://leetcode-cn.com/problems/maximal-rectangle/

• gray-code

  • dp
  • source from:https://leetcode-cn.com/problems/gray-code/

• decode-ways

  • 状态转移方程式dp[i] = dp[i-1]+dp[i-2],复杂在什么时候加前项什么时候加后项，什么时候加两项，0的位置的考虑

  • source from:https://leetcode-cn.com/problems/decode-ways/

• distinct-subsequences

  • 和文本编辑距离的思路很像
  • source from:https://leetcode-cn.com/problems/distinct-subsequences/

• triangle

  • source from:https://leetcode-cn.com/problems/triangle/

• best-time-to-buy-and-sell-stock

• best-time-to-buy-and-sell-stock

  • source from:https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock/

• best-time-to-buy-and-sell-stock-ii

• best-time-to-buy-and-sell-stock-ii

  • source from:https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-ii/

• best-time-to-buy-and-sell-stock-iii

  • source from:https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-iii/

• word-break

  • 第一轮用的traceback，时间复杂度是n的n次方，扑gai；然后我想了下，既然可以考虑回溯，为什么不能递归，其实只要把方程：flag[j] = flag[i]+s[i:j] in wordDict 这个想到就行，这个算是是像加限制性条件的dp，比如20届拼多多校招，多多鸡砍树问题

  • source from:https://leetcode-cn.com/problems/word-break/

• maximum-product-subarray

  • 需要存一个max和min，来做条件判断

  • source from:https://leetcode-cn.com/problems/maximum-product-subarray/

• dungeon-game

  • 反向递归

  • source from:https://leetcode-cn.com/problems/dungeon-game/

• best-time-to-buy-and-sell-stock-iv

  • source from:https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-iv/

• house-robber

  • dp算法很经典的一道题

  • source from:https://leetcode-cn.com/problems/house-robber/

• house-robber-ii

  • source from:https://leetcode-cn.com/problems/house-robber-ii/

• maximal-square

  • source from:https://leetcode-cn.com/problems/maximal-square/

• Ugly Number II

  • source from:https://leetcode-cn.com/problems/ugly-number-ii/

• perfect-squares

• perfect-squares

  • 双循环的dp，同样的思路python会超时，Java不会

  • source from:https://leetcode-cn.com/problems/perfect-squares/

• longest-increasing-subsequence

• longest-increasing-subsequence

  • dp和递归理解的好题目，值得看

  • source from:https://leetcode-cn.com/problems/longest-increasing-subsequence/

• range-sum-query-immutable

  • source from:https://leetcode-cn.com/problems/range-sum-query-immutable/

• best-time-to-buy-and-sell-stock-with-cooldown

  • dp的高难题，代码中带完整带解题方法

  • source from:https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock-with-cooldown/

• burst-balloons

  • dp的超高难题，代码中带完整带解题方法

  • source from:https://leetcode-cn.com/problems/burst-balloons/

• coin-change

  • 斐波纳切动态规划

  • source from:https://leetcode-cn.com/problems/coin-change/

• counting-bits

  • source from:https://leetcode-cn.com/problems/counting-bits/

• integer-break

  • source from:https://leetcode-cn.com/problems/integer-break/

• count-numbers-with-unique-digits

  • source from:https://leetcode-cn.com/problems/count-numbers-with-unique-digits/

• split-array-largest-sum

  • source from:https://leetcode-cn.com/problems/split-array-largest-sum/

• ones-and-zeroes

  • 01背包

  • source from:https://leetcode-cn.com/problems/ones-and-zeroes/

• partition-equal-subset-sum

  • 01背包问题，这题主要考虑提前跳出条件

  • source from:https://leetcode-cn.com/problems/partition-equal-subset-sum/

• target-sum

  • 01背包+递归逻辑

  • source from:https://leetcode-cn.com/problems/target-sum/

• fibonacci-number

  • source from:https://leetcode-cn.com/problems/fibonacci-number/

• longest-palindromic-subsequence

  • 子序列通用双循环dp，这次因为涉及反推，所以用的是倒推法
  • source from:https://leetcode-cn.com/problems/longest-palindromic-subsequence/

• palindromic-substrings

  • 回文子串问题

  • source from:https://leetcode-cn.com/problems/palindromic-substrings/

• Partition to K Equal Sum Subsets

• Partition to K Equal Sum Subsets

  • 依旧是递归，区别在递归逻辑在for循环中，相当于并发了n条处理逻辑，有点像树展开

  • source from:https://leetcode-cn.com/problems/partition-to-k-equal-sum-subsets/

• delete-and-earn

  • 核心在于bitmap+dp，与198题类似

  • source from:https://leetcode-cn.com/problems/delete-and-earn/

• length-of-longest-fibonacci-subsequence

  • 字典进行递归，以后[i,j]作为递归条件判断
  • source from:https://leetcode-cn.com/problems/length-of-longest-fibonacci-subsequence/

• minimum-cost-for-tickets

• minimum-cost-for-tickets

  • source from:https://leetcode-cn.com/problems/minimum-cost-for-tickets/

• valid-permutations-for-di-sequence

  • 爱奇艺2020届校招

  • source from:https://leetcode-cn.com/problems/valid-permutations-for-di-sequence/

• VideoStitching

  • 贪心算法概念的比较好理解的一道题

  • source from:https://leetcode-cn.com/problems/video-stitching/

• longest-string-chain

  • hashmap+简单dp

  • source from:https://leetcode-cn.com/problems/longest-string-chain/

• n-th-tribonacci-number

  • 最简单的dp

  • source from:https://leetcode-cn.com/problems/n-th-tribonacci-number/

• longest-common-subsequence

  • 子序列双循环dp

  • source from:https://leetcode-cn.com/problems/longest-common-subsequence/

• longest-arithmetic-subsequence-of-given-difference

  • 记录状态，和跳跃游戏1一样

  • source from:https://leetcode-cn.com/problems/longest-arithmetic-subsequence-of-given-difference/

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贪心算法

• maximum-length-of-pair-chain

• maximum-length-of-pair-chain

  • source from:https://leetcode-cn.com/problems/maximum-length-of-pair-chain/

---

二叉树

• binary-tree-inorder-traversal

  • 二叉树中序遍历三种写法

  • source from:https://leetcode-cn.com/problems/binary-tree-inorder-traversal/

• binary-tree-level-order-traversal

  • 二叉树层序遍历，利用了队列先进先出的性质

  • source from:https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

• maximum-depth-of-binary-tree

  • 二叉树深度计算，利用了递归性质

  • source from:https://leetcode-cn.com/problems/maximum-depth-of-binary-tree/

• merge-two-binary-trees

  • 二叉树合并，利用了递归性质

  • source from:https://leetcode-cn.com/problems/merge-two-binary-trees/

• diameter-of-binary-tree

  • 结点之间的最大距离，和104题的深度一样

  • source from:https://leetcode-cn.com/problems/diameter-of-binary-tree/

• convert-bst-to-greater-tree

  • 先遍历右子树的中序遍历，面试之前一定要看，易于理解递归在二叉树中的应用

  • source from:https://leetcode-cn.com/problems/convert-bst-to-greater-tree/

• invert-binary-tree

  • 反转树，相互对称

  • source from:https://leetcode-cn.com/problems/invert-binary-tree/

• implement-trie-prefix-tree

  • tire 字典实现

  • source from:https://leetcode-cn.com/problems/implement-trie-prefix-tree/

• construct-binary-tree-from-preorder-and-inorder-traversal

  • 360面试题，真有意思

  • source from:https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

• serialize-and-deserialize-binary-tree

  • 二叉树-->list//list-->二叉树

  • source from:https://leetcode-cn.com/problems/serialize-and-deserialize-binary-tree/

• lowest-common-ancestor-of-a-binary-tree

  • left if right is None else right if left is None else root

  • source from:https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-tree/

• lowest-common-ancestor-of-a-binary-search-tree

  • 和上面一题一摸一样

  • source from:https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

• unique-binary-search-trees

  • 动态规划

  • source from:https://leetcode-cn.com/problems/unique-binary-search-trees/

• symmetric-tree

  • 层次遍历

  • source from:https://leetcode-cn.com/problems/symmetric-tree/

• validate-binary-search-tree

  • 搜索二叉树的左支<结点<右支，正好可以用中序遍历比较

  • source from:https://leetcode-cn.com/problems/validate-binary-search-tree/

• binary-tree-maximum-path-sum

  • 类似104题、543题这种递归的方式

  • source from:https://leetcode-cn.com/problems/binary-tree-maximum-path-sum/

• path-sum-iii

  • dfs+两数之和

  • source from:https://leetcode-cn.com/problems/path-sum-iii/

• unique-binary-search-trees-ii

  • 和unique-binary-search-trees这题思路一样，不同的是，unique-binary-search-trees用的是dp，本题用的是递归，核心和之前一样，[start,i]作为左树去递归，node(i)作为根，[i+1,end]作为右树去递归

  • source from:https://leetcode-cn.com/problems/unique-binary-search-trees-ii/

• same-tree

  • 层序遍历改写

  • source from:https://leetcode-cn.com/problems/same-tree/

• binary-tree-zigzag-level-order-traversal

  • 层序遍历改写

  • source from:https://leetcode-cn.com/problems/binary-tree-zigzag-level-order-traversal/

• recover-binary-search-tree

  • BST+中序遍历的结果是有序的,self.ans.append(root)直接加结点，而非加值了

  • source from:https://leetcode-cn.com/problems/recover-binary-search-tree/

• house-robber-iii

  • 基于树结构+动态规划，可谓是花哨到极点了

  • source from:https://leetcode-cn.com/problems/house-robber-iii/

• construct-binary-tree-from-inorder-and-postorder-traversal

  • 和construct-binary-tree-from-preorder-and-inorder-traversal类似

  • source from:https://leetcode-cn.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/

• construct-binary-tree-from-preorder-and-postorder-traversal

  • 和construct-binary-tree-from-preorder-and-inorder-traversal类似

  • source from:https://leetcode-cn.com/problems/construct-binary-tree-from-preorder-and-postorder-traversal/

• binary-tree-level-order-traversal-ii

  • 层次遍历翻版

  • source from:https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/

• convert-sorted-array-to-binary-search-tree

  • 有序数组-->平衡二叉树：二分查找+递归

  • source from:https://leetcode-cn.com/problems/convert-sorted-array-to-binary-search-tree/

• balanced-binary-tree

  • 判断是否为平衡二叉树，利用的是dfs+maximum-depth-of-binary-tree二叉树深度计算的方法

  • source from:https://leetcode-cn.com/problems/balanced-binary-tree/

• minimum-depth-of-binary-tree

  • source from:https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/

• path-sum

  • source from:https://leetcode-cn.com/problems/path-sum/

  • 上面这两题在return的时候都做了是否是根结点的判断:left if root.right is None else right if root.left is None else left or right

• path-sum-ii

  • dfs

  • source from:https://leetcode-cn.com/problems/path-sum-ii/

• flatten-binary-tree-to-linked-list

  • 有点技巧性

  • source from:https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list/

• populating-next-right-pointers-in-each-node

  • 跨结点直接的指向关系由root.next is not None产生

  • source from:https://leetcode-cn.com/problems/populating-next-right-pointers-in-each-node/

• populating-next-right-pointers-in-each-node-ii

  • 与上一次递归不同，这一次因为不知道root对应的下一层是否有空结点，所以针对上一层的判断结果进行迭代

  • source from:https://leetcode-cn.com/problems/populating-next-right-pointers-in-each-node-ii/

• sum-root-to-leaf-numbers

  • dfs

  • source from:https://leetcode-cn.com/problems/sum-root-to-leaf-numbers/

• binary-tree-preorder-traversal

  • 前序遍历迭代版，技巧就是queue进栈的策略是先右后左，因为前序遍历是中左右，进栈越早出栈越晚

  • source from:https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

• binary-tree-postorder-traversal

  • 巧妙的解法：宽度搜索+逆序出栈==后序输出。另外，这份代码在node = queue.pop()-->node = queue.pop(0),加个0就是层序遍历

  • source from:https://leetcode-cn.com/problems/binary-tree-postorder-traversal/

以上两题加binary-tree-inorder-traversal都是迭代的解法，前序中序写的比较中规中矩，后序写的真的是惊艳

• binary-tree-right-side-view

  • 层序遍历

  • source from:https://leetcode-cn.com/problems/binary-tree-right-side-view/

• add-and-search-word-data-structure-design

  • Tire树很经典的一道题

  • source from::https://leetcode-cn.com/problems/add-and-search-word-data-structure-design/

• count-complete-tree-nodes

  • source from:https://leetcode-cn.com/problems/count-complete-tree-nodes/

• kth-smallest-element-in-a-bst

  • source from:https://leetcode-cn.com/problems/kth-smallest-element-in-a-bst/

• binary-search-tree-iterator

  • 借鉴了上一题中的generation的写法，我觉得非常适合业务中真实使用

  • source from:https://leetcode-cn.com/problems/binary-search-tree-iterator/

• binary-tree-paths

  • dfs

  • source from:https://leetcode-cn.com/problems/binary-tree-paths/

• sum-of-left-leaves

  • 限制性条件下的二叉树遍历计算查找，迭代要比计算更加高效

  • source from:https://leetcode-cn.com/problems/sum-of-left-leaves/

• serialize-and-deserialize-bst

  • 前序遍历保存，二叉搜索树的中序遍历为升序列，两则可以恢复原树

  • source from:https://leetcode-cn.com/problems/serialize-and-deserialize-bst/

• delete-node-in-a-bst

  • 看子节点的是否为None，分情况讨论

  • source from:https://leetcode-cn.com/problems/delete-node-in-a-bst/

• find-mode-in-binary-search-tree

  • 直接遍历统计

  • source from:https://leetcode-cn.com/problems/find-mode-in-binary-search-tree/

• find-bottom-left-tree-value

  • source from:https://leetcode-cn.com/problems/find-bottom-left-tree-value/

• find-largest-value-in-each-tree-row

  • source from:https://leetcode-cn.com/problems/find-largest-value-in-each-tree-row/

• minimum-absolute-difference-in-bst

  • source from:https://leetcode-cn.com/problems/minimum-absolute-difference-in-bst/

• binary-tree-tilt

  • source from:https://leetcode-cn.com/problems/binary-tree-tilt/

• subtree-of-another-tree

  • 递归求解，好题目

  • source from:https://leetcode-cn.com/problems/subtree-of-another-tree/

• most-frequent-subtree-sum

  • 和binary-tree-tilt一样，计算每个结点的子树之和。

  • source from:https://leetcode-cn.com/problems/most-frequent-subtree-sum/

• add-one-row-to-tree

  • 递归直接修改树的实现，爬楼梯的翻版

  • source from:https://leetcode-cn.com/problems/add-one-row-to-tree/

• average-of-levels-in-binary-tree

  • source from:https://leetcode-cn.com/problems/average-of-levels-in-binary-tree/

• find-duplicate-subtrees

  • 套路总结

  • source from:https://leetcode-cn.com/problems/find-duplicate-subtrees/

• two-sum-iv-input-is-a-bst

  • source from:https://leetcode-cn.com/problems/two-sum-iv-input-is-a-bst/

• maximum-binary-tree

  • source from:https://leetcode-cn.com/problems/maximum-binary-tree/

• n-ary-tree-level-order-traversal

  • source from:https://leetcode-cn.com/problems/n-ary-tree-level-order-traversal/

• maximum-depth-of-n-ary-tree

  • source from:https://leetcode-cn.com/problems/maximum-depth-of-n-ary-tree/

• maximum-width-of-binary-tree

  • source from:https://leetcode-cn.com/problems/maximum-width-of-binary-tree/

• trim-a-binary-search-tree

  • 如果接一个root = self.trimBST相当于对结点做一个覆盖，也就是直接修改树

  • source from:https://leetcode-cn.com/problems/trim-a-binary-search-tree/

• second-minimum-node-in-a-binary-tree

  • log(h)的跳出条件的递归写法

  • source from:https://leetcode-cn.com/problems/second-minimum-node-in-a-binary-tree/

• longest-univalue-path

  • 逻辑理解的递归

  • source from:https://leetcode-cn.com/problems/longest-univalue-path/

• n-ary-tree-preorder-traversal

  • source from:https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/

• n-ary-tree-postorder-traversal

  • source from:https://leetcode-cn.com/problems/n-ary-tree-postorder-traversal/

• search-in-a-binary-search-tree

  • source from:https://leetcode-cn.com/problems/search-in-a-binary-search-tree/

• insert-into-a-binary-search-tree

  • source from:https://leetcode-cn.com/problems/insert-into-a-binary-search-tree/

• minimum-distance-between-bst-nodes

  • source from:https://leetcode-cn.com/problems/minimum-distance-between-bst-nodes/

• binary-tree-pruning

  • 先递归到底层，然后在判断是否可以被修剪，修剪条件就是root.val == 0 and not root.left and not root.right:

  • source from:https://leetcode-cn.com/problems/binary-tree-pruning/

• smallest-subtree-with-all-the-deepest-nodes

  • 构造了数据对[root,depth]，用depth判断深度，用root返回depth对应的root

  • source from:https://leetcode-cn.com/problems/smallest-subtree-with-all-the-deepest-nodes/

• leaf-similar-trees

  • source from:https://leetcode-cn.com/problems/leaf-similar-trees/

• all-possible-full-binary-trees

  • 基于树的动态规划，考前必看

  • source from:https://leetcode-cn.com/problems/all-possible-full-binary-trees/

• increasing-order-search-tree

  • source from:https://leetcode-cn.com/problems/increasing-order-search-tree/

• range-sum-of-bst

  • source from:https://leetcode-cn.com/problems/range-sum-of-bst/

• flip-equivalent-binary-trees

  • source from:https://leetcode-cn.com/problems/flip-equivalent-binary-trees/

• check-completeness-of-a-binary-tree

  • 层次遍历，出现空之后不能再出现结点

  • source from:https://leetcode-cn.com/problems/check-completeness-of-a-binary-tree/

• univalued-binary-tree

  • source from:https://leetcode-cn.com/problems/univalued-binary-tree/

• distribute-coins-in-binary-tree

  • 金币题，题目有点难理解，算得是每个结点的多余量，这题和jump-game神似

  • source from:https://leetcode-cn.com/problems/distribute-coins-in-binary-tree/

• cousins-in-binary-tree

  • 这题是结合了父结点+子节点递归，利用depth保存层数，parent保存子结点对应的父结点

  • source from:https://leetcode-cn.com/problems/cousins-in-binary-tree/

• construct-binary-search-tree-from-preorder-traversal

  • source from:https://leetcode-cn.com/problems/construct-binary-search-tree-from-preorder-traversal/

• sum-of-root-to-leaf-binary-numbers

  • ^在python里面是抑或，不是次方

  • source from:https://leetcode-cn.com/problems/sum-of-root-to-leaf-binary-numbers/

• maximum-difference-between-node-and-ancestor

  • source from:https://leetcode-cn.com/problems/maximum-difference-between-node-and-ancestor/

• delete-nodes-and-return-forest

  • 复杂的dfs，在dfs的过程中对树进行修正和保存，有点难

  • source from:https://leetcode-cn.com/problems/delete-nodes-and-return-forest/

• lowest-common-ancestor-of-deepest-leaves

  • source from:https://leetcode-cn.com/problems/lowest-common-ancestor-of-deepest-leaves/

• recover-a-tree-from-preorder-traversal

  • hashmap存结点，每次更新

  • source from:https://leetcode-cn.com/problems/recover-a-tree-from-preorder-traversal/

---

深度遍历

• number-of-islands

  • 回溯/DFS
  • source from:https://leetcode-cn.com/problems/number-of-islands/

• increasing-subsequences

• increasing-subsequences

  • source from:https://leetcode-cn.com/problems/increasing-subsequences/

• generate-parentheses

  • dfs
  • source from:https://leetcode-cn.com/problems/generate-parentheses/

• combination-sum

  • dfs，组合题型1
  • source from:https://leetcode-cn.com/problems/combination-sum/

• combination-sum-ii

  • dfs，组合题型2，index>start保证了双跳跃逻辑，很nice
  • source from:https://leetcode-cn.com/problems/combination-sum-ii/

• subsets-ii

  • dfs，全排列升级版
  • source from:https://leetcode-cn.com/problems/subsets-ii/

• combination-sum-iii

  • dfs，组合题型3
  • source from:https://leetcode-cn.com/problems/combination-sum-iii/

• permutation-sequence

  • dfs,当迭代轮数超过一次完整循环的值的时候，直接跳过
  • source from:https://leetcode-cn.com/problems/permutation-sequence/

• combinations

  • source from:https://leetcode-cn.com/problems/combinations/

• coin-change-2

  • dfs
  • source from:https://leetcode-cn.com/problems/coin-change-2/

• maximum-length-of-a-concatenated-string-with-unique-characters

• maximum-length-of-a-concatenated-string-with-unique-characters

  • dfs
  • source from:https://leetcode-cn.com/problems/maximum-length-of-a-concatenated-string-with-unique-characters/

---

栈

• Valid Parentheses

  • source from:https://leetcode-cn.com/problems/valid-parentheses/

• evaluate-reverse-polish-notation

  • 栈，亮点在python 中对负数取整数部分如何操作

  • source from:https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/

• min-stack

  • 辅助栈，不需要排序，仅需要知道当前最小即可

  • source from:https://leetcode-cn.com/problems/min-stack/

• decode-string

  • 出入栈题，有点难想到

  • source from:https://leetcode-cn.com/problems/decode-string/

• daily-temperatures

  • source from:https://leetcode-cn.com/problems/daily-temperatures/

• remove-all-adjacent-duplicates-in-string

  • source from:https://leetcode-cn.com/problems/remove-all-adjacent-duplicates-in-string/

---

二分搜索

• divide-two-integers

  • 位运算

  • source from:https://leetcode-cn.com/problems/divide-two-integers/

• powx-n

  • 最简单的递归，和快排，斐波纳切数列一致的写法

  • source from:https://leetcode-cn.com/problems/powx-n/

• find-minimum-in-rotated-sorted-array

• find-minimum-in-rotated-sorted-array

  • source from:https://leetcode-cn.com/problems/find-minimum-in-rotated-sorted-array/

• find-minimum-in-rotated-sorted-array-ii

• find-minimum-in-rotated-sorted-array-ii

  • source from:https://leetcode-cn.com/problems/find-minimum-in-rotated-sorted-array-ii/

• Searcha2DMatrixII

  • 这题其实可以从右上角开始查找，会更快

  • source from::https://leetcode.com/problems/search-a-2d-matrix-ii/

• IntersectionofTwoArrays

  • source from:https://leetcode-cn.com/problems/intersection-of-two-arrays/

• split-array-largest-sum

  • 二分搜索，亮点在分组最大和变成了原数组的[max(原数组),sum(原数组))

  • source from:https://leetcode-cn.com/problems/split-array-largest-sum/

• binary-search

  • source from:https://leetcode-cn.com/problems/binary-search/

• koko-eating-bananas

• koko-eating-bananas

  • source from:https://leetcode-cn.com/problems/koko-eating-bananas/

---

回溯

• Letter Combinations of a Phone Number

• LetterCombinationsofaPhoneNumber

  • 这题用了dp和递归两种方法，dp更好理解也很Pythonic，递归则更加通用

  • 盖题递归位置在循环中，很有意思

  • source from:https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/

• permutations

  • 递归

  • source from:https://leetcode-cn.com/problems/permutations/

• permutations-ii

  • 简单的回溯算法

  • source from:https://leetcode-cn.com/problems/permutations-ii/

• palindrome-partitioning

  • 地址和值；已经用ansValue用的是内存地址，时刻在变，所以要new ArrayList(ansValue)固定值使其不变

  • 回溯的剔除；

  • source from:https://leetcode-cn.com/problems/palindrome-partitioning/

• additive-number

  • source from:https://leetcode-cn.com/problems/additive-number/

• split-array-into-fibonacci-sequence

  • source from:https://leetcode-cn.com/problems/split-array-into-fibonacci-sequence/

• palindrome-pairs

  • source from:https://leetcode-cn.com/problems/palindrome-pairs/

---

链表

• AddTwoNumbers

  • source from:https://leetcode-cn.com/problems/add-two-numbers/

• remove-nth-node-from-end-of-list

  • 链表很经典的一道题

  • source from::https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/

• Merge Two Sorted Lists

  • source from:https://leetcode-cn.com/problems/merge-two-sorted-lists/

• Swap Nodes in Pairs

  • 递归算法，讲的很详细，递归复习专用

  • 该题递归在函数中间位置，很有意思

  • source from:https://leetcode-cn.com/problems/swap-nodes-in-pairs/

• RemoveLinkedListElements

  • 协助理解链表里面的head = head, head.next = head.next.next

  • source from:https://leetcode-cn.com/problems/remove-linked-list-elements/

• reverse-linked-list

  • 递归在中间,简单题，对比上面的24题可以对比着看

  • source from:https://leetcode-cn.com/problems/reverse-linked-list/

• reverse-nodes-in-k-group

  • 翻转链表用的是迭代，参考206题reverse-linked-list
  • 交换参考的是24题Swap Nodes in Pairs
  • source from:https://leetcode-cn.com/problems/reverse-nodes-in-k-group/
  • 深入理解地址和值

• palindrome-linked-list

  • source from:https://leetcode-cn.com/problems/palindrome-linked-list/

• linked-list-cycle

  • 快慢指针，类似题型find-the-duplicate-number
  • source from:https://leetcode-cn.com/problems/linked-list-cycle/

• linked-list-cycle-ii

  • 快慢指针，类似题型find-the-duplicate-number
  • source from:https://leetcode-cn.com/problems/linked-list-cycle-ii/

• intersection-of-two-linked-lists

  • source from:https://leetcode-cn.com/problems/intersection-of-two-linked-lists/
  • 短链表+长链表<-->长链表+短链表，同等初始的结尾即为交点

• sort-list

  • source from:https://leetcode-cn.com/problems/sort-list/
  • 并归排序+链表

• merge-k-sorted-lists

  • source from:https://leetcode-cn.com/problems/merge-k-sorted-lists/
  • 和上一个很像，并归排序+链表
  • 快排按照标值二分，小的在前大的在后，而归并则是按照下标二分，再分别对两个部分归并排序，先分后和，在和的过程中排序

• rotate-list

  • source from:https://leetcode-cn.com/problems/rotate-list/

• remove-duplicates-from-sorted-list-ii

  • source from:https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list-ii/
  • 建立一个哨兵指针，每次对比cur和cur.next的val是否一致

• remove-duplicates-from-sorted-list

  • source from:https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list/
  • 上面一条的简单版本

• partition-list

  • source from:https://leetcode-cn.com/problems/partition-list/

• reverse-linked-list-ii

  • source from:https://leetcode-cn.com/problems/reverse-linked-list-ii/
  • prev.next为反转后的尾结点，正好可以接剩余结点，非常精彩

• convert-sorted-list-to-binary-search-tree

  • source from:https://leetcode-cn.com/problems/convert-sorted-list-to-binary-search-tree/
  • 反转链表reverse-linked-list + 平衡二叉树balanced-binary-tree

• reorder-list

  • source from:https://leetcode-cn.com/problems/reorder-list/
  • 反转链表reverse-linked-list

• insertion-sort-list

  • source from:https://leetcode-cn.com/problems/insertion-sort-list/
  • 这题和remove-duplicates-from-sorted-list-ii类似，需要不停的从head结点找到比当前结点.next.val要小的结点位置
  • 链表删结点方法

• delete-node-in-a-linked-list

  • source from:https://leetcode-cn.com/problems/delete-node-in-a-linked-list/
  • 一行但是非常技巧

• odd-even-linked-list

  • source from:https://leetcode-cn.com/problems/odd-even-linked-list/

• add-two-numbers-ii

  • source from:https://leetcode-cn.com/problems/add-two-numbers-ii/
  • 双栈+链表

• split-linked-list-in-parts

  • source from:https://leetcode-cn.com/problems/split-linked-list-in-parts/

• flatten-a-multilevel-doubly-linked-list

  • source from:https://leetcode-cn.com/problems/flatten-a-multilevel-doubly-linked-list/

• linked-list-components

  • source from:https://leetcode-cn.com/problems/linked-list-components/

• linked-list-components

  • source from:https://leetcode-cn.com/problems/linked-list-components/

• middle-of-the-linked-list

  • source from:https://leetcode-cn.com/problems/middle-of-the-linked-list/

• next-greater-node-in-linked-list

  • source from:https://leetcode-cn.com/problems/next-greater-node-in-linked-list/
  • 栈

• remove-zero-sum-consecutive-nodes-from-linked-list

  • source from:https://leetcode-cn.com/problems/remove-zero-sum-consecutive-nodes-from-linked-list/
  • 前缀和+递归，面试前必看

---

堆

• kth-largest-element-in-an-array

  • 加速的地方在于，维护好容量为k的小根堆之后，只需要保证根堆的根结点最小即可。min_heap(k_min, k, 0)

  • source from:https://leetcode-cn.com/problems/kth-largest-element-in-an-array/

• sliding-window-maximum

  • 双端队列题目，用队列保留一个递减序列

  • source from:https://leetcode-cn.com/problems/sliding-window-maximum/

• top-k-frequent-elements

  • 求最大：小根堆；求最小：大根堆。因为只需要保证最大最小的k个值在堆内即可，无需遍历堆。 根堆输出的时候，每次交换根堆根结点到最后，小根堆的化，数组越靠后越小，相当于降序排列；大根堆的话，数组越往后越大，相当于升序排列。

  • source from:https://leetcode-cn.com/problems/top-k-frequent-elements/

• DistantBarcodes

  • source from:https://leetcode-cn.com/problems/distant-barcodes/

---

双指针

• LongestSubstringWithoutRepeatingCharacters

  • 滑动窗口

  • source from:https://leetcode-cn.com/problems/longest-substring-without-repeating-characters/

• summary-ranges

• summary-ranges

  • 数组当做双指针的一种方式

  • source from:https://leetcode-cn.com/problems/summary-ranges/

• boats-to-save-people

  • 双指针

  • source from:https://leetcode-cn.com/problems/boats-to-save-people/

• fruit-into-baskets

  • 双指针

  • source from:https://leetcode-cn.com/problems/fruit-into-baskets/

• minimum-window-substring

  • 双指针+滑动窗口，模板要记得

  • source from:https://leetcode-cn.com/problems/minimum-window-substring/

• is-subsequence

  • 双指针+滑动窗口，模板要记得

  • source from:https://leetcode-cn.com/problems/is-subsequence/

• minimum-size-subarray-sum

  • 子集问题，双指针

  • source from:https://leetcode-cn.com/problems/minimum-size-subarray-sum/

• longest-chunked-palindrome-decomposition

  • 回文问题，双指针

  • source from:https://leetcode-cn.com/problems/longest-chunked-palindrome-decomposition/

• shortest-palindrome

  • 回文问题，双指针

  • source from:https://leetcode-cn.com/problems/shortest-palindrome/

• subarray-product-less-than-k

• subarray-product-less-than-k

  • 区间边界解决重复，双指针

  • source from:https://leetcode-cn.com/problems/subarray-product-less-than-k/

• max-consecutive-ones-iii

• max-consecutive-ones-iii

  • source from:https://leetcode-cn.com/problems/max-consecutive-ones-iii/

• replace-the-substring-for-balanced-string

  • 最优子串，双指针

  • source from:https://leetcode-cn.com/problems/replace-the-substring-for-balanced-string/

---

字符串

• Roman to Integer

  • source from:https://leetcode-cn.com/problems/roman-to-integer/

• Longest Common Prefix

  • source from:https://leetcode-cn.com/problems/longest-common-prefix/

• detect-capital

  • source from:https://leetcode-cn.com/problems/detect-capital/

• custom-sort-string

  • source from:https://leetcode-cn.com/problems/custom-sort-string/

• goat-latin

  • source from:https://leetcode-cn.com/problems/goat-latin/

• orderly-queue

  • 有点冒泡排序的意思

  • source from:https://leetcode-cn.com/problems/orderly-queue/

• ReverseOnlyLetters

  • source from:https://leetcode-cn.com/problems/reverse-only-letters/

• longest-uncommon-subsequence-i

  • source from:https://leetcode-cn.com/problems/longest-uncommon-subsequence-i/

• longest-continuous-increasing-subsequence

  • source from:https://leetcode-cn.com/problems/longest-continuous-increasing-subsequence/

• sort-characters-by-frequency

• sort-characters-by-frequency

  • source from:https://leetcode-cn.com/problems/sort-characters-by-frequency/

---

递归

• reverse-string-ii

• reverse-string-ii

  • source from:https://leetcode-cn.com/problems/reverse-string-ii/

• K-thSymbolinGrammar

• K-th Symbol in Grammar

  • python用了最简单的递归表示，把结果表示出来再做，比较慢,所以加了@functools.lru_cache(maxsize=128, typed=False)对结果进行缓存

  • source from:https://leetcode-cn.com/problems/k-th-symbol-in-grammar/

• valid-tic-tac-toe-state

  • 直接罗列全情况，排除一下就行

  • source from:https://leetcode-cn.com/problems/valid-tic-tac-toe-state/

• valid-palindrome-ii

  • source from:https://leetcode-cn.com/problems/valid-palindrome-ii/

---

蓄水池抽样

• random-pick-index

  • 蓄水池抽烟，randint(0,cnt)控制了每次被抛弃的概率为1/(1+cnt）

  • source from:https://leetcode-cn.com/problems/random-pick-index/

• linked-list-random-node

  • source from:https://leetcode-cn.com/problems/linked-list-random-node/

---

数学

• reverse Integera

  • source from:https://leetcode-cn.com/problems/reverse-integer/

• Palindrome Number

  • source from:https://leetcode-cn.com/problems/palindrome-number/

• ExcelSheetColumnNumber

  • source from:https://leetcode-cn.com/problems/excel-sheet-column-number/

• UglyNumber

  • source from:https://leetcode-cn.com/problems/ugly-number/

• reach-a-number

  • source from:https://leetcode-cn.com/problems/reach-a-number/

• ChalkboardXORGame

  • 逻辑鬼才的题目

  • source from:https://leetcode-cn.com/problems/chalkboard-xor-game/

• LargestTriangleArea

  • 任给三点，三角形面积等于abs(x1*(y2-y3)+x2*(y3-y1)+x3*(y1-y2))/2

  • source from:https://leetcode-cn.com/problems/largest-triangle-area/

• consecutive-numbers-sum

  • (n-m)(n+m+1)=2N,a=n-m,b=n+m+1,a+b=2n+1,b-a=2m+1,所以a*b=2N,a,b一奇一偶

  • source from:https://leetcode-cn.com/problems/consecutive-numbers-sum/

• surface-area-of-3d-shapes

  • 透视图角度

  • source from:https://leetcode-cn.com/problems/surface-area-of-3d-shapes/

• greatest-common-divisor-of-strings

  • gcd，最大公约数；辗转相除法复习

  • source from:https://leetcode-cn.com/problems/greatest-common-divisor-of-strings/

• prime-palindrome

  • 素数+回文
  • source from:https://leetcode-cn.com/problems/prime-palindrome/

• find-the-closest-palindrome

  • 罗列情况
  • source from:https://leetcode-cn.com/problems/find-the-closest-palindrome/

• repeated-substring-pattern

  • 2N-2>N
  • source from:https://leetcode-cn.com/problems/repeated-substring-pattern/

• sum-of-square-numbers

• sum-of-square-numbers

  • 平方数之和
  • 二分查找
  • source from:https://leetcode-cn.com/problems/sum-of-square-numbers/

---

位运算

• SingleNumber

  • 这题有数学解法和位解法两种，都很值得看一下

  • source from:https://leetcode.com/problems/single-number/description/

• single-number-ii

• single-number-ii

  • 知识点1：设计一个逻辑运算，使满足存在两个变量a和b，当遍历nums的时候，对于重复元素x，第一次碰到x的时候，我们会将x赋给a，第二次碰到后再赋给b，第三次则全量抵消

  • 知识点2：x&~x=0

  • source from:https://leetcode-cn.com/problems/single-number-ii/

• reverse-bits

  • source from:https://leetcode-cn.com/problems/reverse-bits/

• sum-of-two-integers

  • source from:https://leetcode-cn.com/problems/sum-of-two-integers/

• hamming-distance

  • source from:https://leetcode-cn.com/problems/hamming-distance/

---

哈希表

• JewelsandStones

  • source from:https://leetcode-cn.com/problems/jewels-and-stones/

• find-all-anagrams-in-a-string

  • 滑动窗口去解决字符串匹配问题

  • source from:https://leetcode-cn.com/problems/find-all-anagrams-in-a-string/

• distribute-candies

• distribute-candies

  • source from:https://leetcode-cn.com/problems/distribute-candies/

• uncommon-words-from-two-sentences

  • source from:https://leetcode-cn.com/problems/uncommon-words-from-two-sentences/

• substring-with-concatenation-of-all-words

  • hash对比
  • source from:https://leetcode-cn.com/problems/substring-with-concatenation-of-all-words/

• longest-harmonious-subsequence

  • source from:https://leetcode-cn.com/problems/longest-harmonious-subsequence/

• number-of-matching-subsequences

  • source from:https://leetcode-cn.com/problems/number-of-matching-subsequences/

• n-repeated-element-in-size-2n-array

• n-repeated-element-in-size-2n-array

• n-repeated-element-in-size-2n-array

  • source from:https://leetcode-cn.com/problems/n-repeated-element-in-size-2n-array/

---

贪心算法

• gas-station

• gas-station

  • source from:https://leetcode-cn.com/problems/gas-station/

• candy

  • 转转2020届校招

  • 分发糖果，左右各扫一遍取max

  • source from:https://leetcode-cn.com/problems/candy/

• queue-reconstruction-by-height

  • 原题说的是前面大于等于当前身高的，所以插矮个子不会影响排序

  • source from:https://leetcode-cn.com/problems/queue-reconstruction-by-height/

• two-city-scheduling

  • source from:https://leetcode-cn.com/problems/two-city-scheduling/

• LemonadeChange

  • source from:https://leetcode-cn.com/problems/lemonade-change/

---

分治算法

• beautiful-array

• beautiful-array

  • source from:https://leetcode-cn.com/problems/beautiful-array/

---

桶排序

• maximum-gap

  • 分桶，亮点在于分N+1个桶，保证一定有一个空桶，空桶的存在保证最大间隔在桶之间，不需要再去比桶内值

  • source from:https://leetcode-cn.com/problems/maximum-gap/

---

其他

• NimGame

  • 规律题，数学归纳

  • source from:https://leetcode-cn.com/problems/nim-game/

• LexicographicalNumbers

  • 字典排序，用了python特性，也同时整理基础概念，值得去看

  • source from:https://leetcode-cn.com/problems/lexicographical-numbers/

• IncreasingTripletSubsequence

  • such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return false,其中只要保证固定住最小和最大，不断降低最小值的下限，使得比最小值要大的值可以尽可能的接近最小值，从而更容易找到比接近最小值的值

  • source from:https://leetcode-cn.com/problems/increasing-triplet-subsequence/

• relative-ranks

  • source from:https://leetcode-cn.com/problems/relative-ranks/

• largest-number

  • 这题技巧题，理解sorted中的cmp和key的作用，key是sorted元素判断依据的接受入口，而接收值需要调用functools中的cmp_to_key进行包装，python2不用这么复杂

  • source from:https://leetcode-cn.com/problems/largest-number/

• lru-cache

  • source from:https://leetcode-cn.com/problems/lru-cache/