250. Count Univalue Subtrees

 1 public class Solution {
 2     int cnt = 0;
 3     public int countUnivalSubtrees(TreeNode root) {
 4         helper(root);
 5         return cnt;
 6     }
 7     
 8     private boolean helper(TreeNode root) {
 9         if(root == null) {
10             return true;
11         }
12         boolean left = helper(root.left);
13         boolean right = helper(root.right);
14         if(left && right) {
15             if(root.left != null && root.val != root.left.val) {
16                 return false;
17             }
18             if(root.right != null && root.val != root.right.val) {
19                 return false;
20             }
21             cnt++;
22             return true;
23         }
24         return false;
25     }
26 }