LeetCode 1973. Count Nodes Equal to Sum of Descendants（DFS）

Input: root = [10,3,4,2,1]
Output: 2
Explanation:
For the node with value 10: The sum of its descendants is 3+4+2+1 = 10.
For the node with value 3: The sum of its descendants is 2+1 = 3.

Input: root = [2,3,null,2,null]
Output: 0
Explanation:
No node has a value that is equal to the sum of its descendants.

Input: root = [0]
Output: 1
For the node with value 0: 
The sum of its descendants is 0 since it has no descendants.
 

Constraints:
The number of nodes in the tree is in the range [1, 10^5].
0 <= Node.val <= 10^5

/**
 * 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 equalToDescendants(TreeNode* root) {
        dfs(root);
        return ans;
    }
    long long dfs(TreeNode* root)
    {
        if(!root) return 0;
        auto l = dfs(root->left);
        auto r = dfs(root->right);
        if(root->val == l+r)
            ans++;
        return l+r+root->val;
    }
};