LeetCode Count Complete Tree Nodes

/**
 * 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:
    int countNodes(TreeNode* root) {
        return count_nodes(root);
    }
    
    int count_nodes(TreeNode* root) {
        if (root == NULL) {
            return 0;
        }
        int cnt = 1;
        cnt += count_nodes(root->left) + count_nodes(root->right);
        return cnt;
    }
};