[leetcode]250. Count Univalue Subtrees统计节点值相同的子树

Given a binary tree, count the number of uni-value subtrees.

A Uni-value subtree means all nodes of the subtree have the same value.

Example :

Input:  root = [5,1,5,5,5,null,5]

              5
             / 
            1   5
           /    
          5   5   5

Output: 4

题意：

给定一棵二叉树，求所有节点都同值的二叉树。

思路：

dfs

代码：

class Solution {
    int result; 
    public int countUnivalSubtrees(TreeNode root) {
        result = 0; 
        helper(root); 
        return result;
    }
     
    private boolean helper(TreeNode node){
        if(node == null ) return true;
        
        boolean left = helper(node.left);
        boolean right = helper(node.right);
        
        if(left&&right){
        if ((node.left!= null && node.val != node.left.val) || (node.right!= null && node.val != node.right.val) ){
                return false;
            }
            result++;
            return true;
        } 
        return false;
    }
}