一、题目

A binary tree is uni-valued if every node in the tree has the same value.

Given the root of a binary tree, return true if the given tree is uni-valued, or false otherwise.

Example 1:

Input: root = [1,1,1,1,1,null,1]
Output: true
Example 2:

Input: root = [2,2,2,5,2]
Output: false

Constraints:

The number of nodes in the tree is in the range [1, 100].
0 <= Node.val < 100

二、题解

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:TreeNode* pre;bool isUnivalTree(TreeNode* root) {if(root == nullptr) return true;if(pre != nullptr){if(pre->val != root->val) return false;}pre = root;bool leftUni = isUnivalTree(root->left);bool rightUni = isUnivalTree(root->right);return leftUni && rightUni;}
};