250

/**
 * 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 {
int ans = 0;
public:
    int countUnivalSubtrees(TreeNode* root) {
        dfs(root);
        return ans;
    }

    bool dfs(TreeNode* root){
        if(!root) return true;
        if(!root->left && !root->right){
            ans++;
            return true;
        }
        bool sameVal = true;
        if(root->left){
            sameVal &= dfs(root->left) && (root->val == root->left->val);
        }
        if(root->right){
            sameVal &= dfs(root->right) && (root->val == root->right->val);
        }

        if(!sameVal) return false;
        ans++;
        return true;
    }
};