261. Graph Valid Tree

也是卡了好多天的题目

主要就是介绍了union-find的算法，用于检查Undirected graph有没有环

http://www.geeksforgeeks.org/union-find/

 1     public boolean validTree(int n, int[][] edges) {
 2         int[] parent = new int[n];
 3         Arrays.fill(parent, -1);
 4         for(int i = 0; i < edges.length ; i++) {
 5             int x = find(parent, edges[i][0]);
 6             int y = find(parent, edges[i][1]);
 7             if(x == y) {
 8                 return false;
 9             }
10             parent[x] = y;
11         }
12         return edges.length == n - 1;
13     }
14
15     private int find(int[] parent, int i) {
16         if(parent[i] == -1) {
17             return i;
18         }
19         return find(parent, parent[i]);
20     }