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

class Solution {  
    private int count;
    public int countUnivalSubtrees(TreeNode root) {  
        uni(root);  
        return count;  
    }  
      
    boolean uni(TreeNode root) {  
        if(root == null)  
            return true;  
        if(root.left ==null && root.right == null) {  
            count++;  
            return true;  
        }  
        bool left = uni(root.left);  
        bool right = uni(root.right);  
        if(left && right && (root.left == null || root.left.val == root.val) && (root.right == null || root.right.val == root.val)) {  
            count++;  
            return true;  
        }  
        return false;  
    }  
}