Maximum Depth of Binary Tree

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
     int max=-1;
     void meng(TreeNode*root,int dep)
     {
         if((root->left==NULL)&&(root->right==NULL))
         {
             if(dep>max)
                 max=dep;
             return;
         }
         if(root->left)
             meng(root->left,dep+1);
         if(root->right)
             meng(root->right,dep+1);
     }
    int maxDepth(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         max=-1;
         if(root==NULL)
             return 0;
         meng(root,1);
         return max;
    }
};