/** * 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;        }        int dl = maxDepth(root->left);        int dr = maxDepth(root->right);        return max(dl, dr) + 1;    }};