[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;
     }
 }