178. Graph Valid Tree

Description

Given n nodes labeled from 0 to n - 1 and a list of undirected edges (each edge is a pair of nodes), write a function to check whether these edges make up a valid tree.

You can assume that no duplicate edges will appear in edges. Since all edges are undirected, [0, 1] is the same as [1, 0] and thus will not appear together in edges.

Example

Example 1:

Input: n = 5 edges = [[0, 1], [0, 2], [0, 3], [1, 4]]

Output: true.

Example 2:

Input: n = 5 edges = [[0, 1], [1, 2], [2, 3], [1, 3], [1, 4]]

Output: false.

思路：

判断一个图是二叉树的条件：

1.边比顶点少1

2.BFS遍历一遍的结果覆盖了全部的点。

注意defaultdict的应用，可以给定默认值而不报错。

代码：