leetcode-java/src/ValidateBinarySearchTree98.java at master · fluency03/leetcode-java · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
/**
 * Given a binary tree, determine if it is a valid binary search tree (BST).
 *
 * Assume a BST is defined as follows:
 *
 * The left subtree of a node contains only nodes with keys less than the node's key.
 * The right subtree of a node contains only nodes with keys greater than the node's key.
 * Both the left and right subtrees must also be binary search trees.
 *
 * Example 1:
 * 2
 * / \
 * 1 3
 * Binary tree [2,1,3], return true.
 *
 * Example 2:
 * 1
 * / \
 * 2 3
 * Binary tree [1,2,3], return false.
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */

publicclassValidateBinarySearchTree98 {
publicbooleanisValidBST(TreeNoderoot) {
returnhelper(root, Long.MIN_VALUE, Long.MAX_VALUE);
 }

privatebooleanhelper(TreeNoderoot, longmin, longmax) {
if (root == null) returntrue;
if (root.val <= min || root.val >= max) returnfalse;
returnhelper(root.left, min, root.val) && helper(root.right, root.val, max);
 }


publicbooleanisValidBST2(TreeNoderoot) {
if (root == null) returntrue;
returnisValidBST(root, newint[]{0, 0});
 }

privatebooleanisValidBST(TreeNoderoot, int[] bounds) {
bounds[0] = root.val;
bounds[1] = root.val;
if (root.left == null && root.right == null) {
returntrue;
 }

if (root.left != null) {
int[] leftBounds = newint[]{0, 0};
booleanleft = isValidBST(root.left, leftBounds);
if (!left || leftBounds[1] >= root.val) returnfalse;
bounds[0] = leftBounds[0];
 }

if (root.right != null) {
int[] rightBounds = newint[]{0, 0};
booleanright = isValidBST(root.right, rightBounds);
if (!right || rightBounds[0] <= root.val) returnfalse;
bounds[1] = rightBounds[1];
 }

returntrue;
 }


/**
 * https://discuss.leetcode.com/topic/46016/learn-one-iterative-inorder-traversal-apply-it-to-multiple-tree-questions-java-solution
 */
publicbooleanisValidBST3(TreeNoderoot) {
if (root == null) returntrue;
Stack<TreeNode> stack = newStack<>();
TreeNodepre = null;
while (root != null || !stack.isEmpty()) {
while (root != null) {
stack.push(root);
root = root.left;
 }
root = stack.pop();
if(pre != null && root.val <= pre.val) returnfalse;
pre = root;
root = root.right;
 }
returntrue;
 }

}