1 public class Solution { 2     private int[] parent; 3     public boolean validTree(int n, int[][] edges) { 4         if (n < 2) { 5             return true; 6         } 7         parent = new int[n]; 8         for (int i = 0; i < n; i++) { 9             parent[i] = i;10         }11         12         for (int[] edge : edges) {13             int indexA = findParent(edge[0]);14             int indexB = findParent(edge[1]);15             if (indexA == indexB) {16                 return false;17             }18             if (indexA < indexB) {19                 parent[indexB] = indexA;20             } else if (indexA > indexB) {21                 parent[indexB] = parent[indexA];22             }23         }24         int count = 0;25         for (int i = 0; i < n; i++) {26             if (parent[i] == i) {27     private int findParent(int i) {28         while (i != parent[i]) {29             parent[i] = parent[parent[i]];30             i = parent[i];31         }32         return i;33     }34 }

1. No circle : parent index not equal

2. Only one root.