LeetCode965. Univalued Binary Tree

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;
    }
};