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)
14             return 0;
15         return max(maxDepth(root->left), maxDepth(root->right))+1;
16     }
17 private:
18     int max(int a, int b) {
19         return a>b?a:b;
20     }
21 };