/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int maxDepth(TreeNode *root) {        if (root == NULL) return 0;        return max(maxDepth(root->left), maxDepth(root->right)) + 1;    }};