leetcode 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 max;

 void deepSearch(TreeNode * root, int depth)
 {
     
     if (root != NULL)
     {
         deepSearch(root->left, depth + 1);
         deepSearch(root->right, depth + 1);
     }
     else
     {
         if (depth > max)
         {
             max = depth;
         }
     }
 }

 int maxDepth(struct TreeNode* root) 
 {
     if(root==NULL)
     return 0;
     max = 0;
     deepSearch(root, 0);
     return max;
 }
};