graph_valid_tree.py

# -*- coding: utf-8 -*-

class Solution:
 # @param {int} n an integer
 # @param {int[][]} edges a list of undirected edges
 # @return {boolean} true if it's a valid tree, or false
 def validTree(self, n, edges):
 # Write your code here
 # 如果是树，n个点有n - 1条边，遍历后包含所有点。
 if len(edges) != (n - 1):
 return False
 # 重新构造无向树，[m, n] => t[m][n] = t[n][m] = True。
 tree = [[False] * n for i in xrange(n)]
 visited = [False] * n
 for edge in edges:
 tree[edge[0]][edge[1]] = True
 tree[edge[1]][edge[0]] = True
 # 从节点0开始深度遍历，这里顺便解决了输入为"1, []"的情况。
 self.dfs(tree, visited, 0)
 for i in xrange(n):
 if not visited[i]:
 return False
 return True

 def dfs(self, tree, visited, node):
 visited[node] = True
 for i in xrange(len(tree[node])):
 if tree[node][i] and (not visited[i]):
 self.dfs(tree, visited, i)