Maximum Depth of Binary Tree

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(root==NULL) return 0;
        return max(dep(root->left,1),dep(root->right,1));
    }
    int dep(TreeNode* root,int pre_h){
        if(root==NULL) return pre_h;
        if(root->left ==NULL && root->right ==NULL) return pre_h+1;
        return max(dep(root->left,pre_h+1),dep(root->right,pre_h+1));
    }
};