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 {
private:
    int maxdepth;
public:
    void solve(TreeNode *root,int depth){
        if(root==NULL)
            return;
        if(depth>maxdepth)
            maxdepth=depth;
        solve(root->left,depth+1);
        solve(root->right,depth+1);
    }
    int maxDepth(TreeNode *root) {
        maxdepth=0;
        solve(root,1);
        return maxdepth;
    }
};