leetcode 250 Count Univalue Subtrees

思路很简单，存在最优子结构，用递归自上至下解即可。

如何判断当前root所代表的树是否为uni-value subtree?

1. 检查root和left，right值是否一致

2. left子树和right子树是否都为uni-value

注意第二步，可能left，right为空，写if时要注意

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