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:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int countUnivalSubtrees(TreeNode root) {
        int [] res = {0};
        dfs(root, res);
        return res[0];
    }
    
    private boolean dfs(TreeNode root, int [] res){
        if(root == null){
            return true;
        }
        
        boolean left = dfs(root.left, res);
        boolean right = dfs(root.right, res);
        if(left && right){
            if(root.left!=null && root.left.val!=root.val){
                return false;
            }
            
            if(root.right!=null && root.right.val!=root.val){
                return false;
            }
            
            res[0]++;
            return true;
        }
        
        return false;
    }
}

类似Longest Univalue Path.