965. Univalued Binary Tree : https://leetcode.com/problems/univalued-binary-tree/

Univalued Binary Tree.cc

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