LeetCode 250. Count Univalue Subtrees

原题链接在这里：https://leetcode.com/problems/count-univalue-subtrees/

题目：

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.

For example:
Given binary tree,

              5
             / \
            1   5
           / \   \
          5   5   5 

return 4.

题解：

bottom-up recursion. dfs返回当前root下是不是univalue tree.

Time Complexity: O(n).

Space: O(logn). height of tree.

AC Java:

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution {
11     public int countUnivalSubtrees(TreeNode root) {
12         int [] res = {0};
13         dfs(root, res);
14         return res[0];
15     }
16     
17     private boolean dfs(TreeNode root, int [] res){
18         if(root == null){
19             return true;
20         }
21         
22         boolean left = dfs(root.left, res);
23         boolean right = dfs(root.right, res);
24         if(left && right){
25             if(root.left!=null && root.left.val!=root.val){
26                 return false;
27             }
28             
29             if(root.right!=null && root.right.val!=root.val){
30                 return false;
31             }
32             
33             res[0]++;
34             return true;
35         }
36         
37         return false;
38     }
39 }

类似Longest Univalue Path.