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