/**
 * 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

        return dfsHelper(root);
    }
    int dfsHelper(TreeNode* root) {
        if (root == NULL) {
            return 0;
        }
        int maxNum = max(dfsHelper(root->left), dfsHelper(root->right)) + 1;
        return maxNum;
    }
};