LeetCode Count Univalue Subtrees

Posted 2020-06-13

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

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.

helper funtion 来返回当前tree 是不是 unitree.

两个终止条件一个不能少，若root == null 返回true. 若root是叶子节点, res[0]++, 返回true.

recursion时left保存左子树是否是unitree, right保存右子树是否是unitree.

若都是检查root的val 和左右是否相等，

若想等, res[0]++, 返回true.

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 public class Solution {
11     public int countUnivalSubtrees(TreeNode root) {
12         int [] res = {0};
13         helper(root, res);
14         return res[0];
15     }
16     
17     private boolean helper(TreeNode root, int [] res){
18         if(root == null){
19             return true;
20         }
21         
22         if(root.left == null && root.right == null){
23             res[0]++;
24             return true;
25         }
26         
27         boolean left = helper(root.left, res);
28         boolean right = helper(root.right, res);
29         if(left && right && (root.left == null || root.left.val == root.val) && (root.right == null || root.right.val == root.val)){
30             res[0]++;
31             return true;
32         }
33         return false;
34     }
35 }