/**

 * Definition of TreeNode:

 * class TreeNode {

 * public:

 *     int val;

 *     TreeNode *left, *right;

 *     TreeNode(int val) {

 *         this->val = val;

 *         this->left = this->right = NULL;

 *     }

 * }

 */

class Solution {

public:

    /**

     * @param root: The root of binary tree.

     * @return: An integer

     */

    int maxDepth(TreeNode * root) {

        // write your code here

        if (!root) return ;

        return  + max(maxDepth(root->left), maxDepth(root->right));

    }

};