1 /** 2  * Definition for binary tree 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     int maxDepth(TreeNode *root) {13         if (!root) return 0;14         return max(maxDepth(root->left), maxDepth(root->right)) + 1;15     }16 };