1 /** 2  * Definition for a binary tree node. 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 == NULL){14             return 0;15         }16         17         int leftDepth = maxDepth(root->left);18         int rightDepth = maxDepth(root->right);19         20         return max(leftDepth, rightDepth)+1;21     }22 };