965. Univalued Binary Tree

1. Description

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.

2. Example

Example 1:

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

Example 2:

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

3. Constraints

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

4. Solutions

Depth-First Search

n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    bool isUnivalTree(TreeNode *root) {
        return root == nullptr ||
            (isUnivalTree(root->left) && isUnivalTree(root->right) &&
             (root->left != nullptr ? root->val == root->left->val : true) &&
             (root->right != nullptr ? root->val == root->right->val : true));
    }
};