[LeetCode] 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) {}
 * };
 */
 int maxP(TreeNode *p,int len){
     if(p==NULL)
       return len;
     else {
         int a = maxP(p->left,len+1);
         int b = maxP(p->right,len+1);
         return a>b ?  a:b ;
     }
}
class Solution {
public:
    int maxDepth(TreeNode *root) {
        int len=0;
        len = maxP(root,len);
        return len;
    }
};