LeetCode-Unique Binary Search Trees

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

For example,
Given n = 3, there are a total of 5 unique BST's.

   1         3     3      2      1    \       /     /      / \      \     3     2     1      1   3      2    /     /       \                 \   2     1         2                 3

Solution:

Code:

<span style="font-size:14px;">class Solution {public:    int helper(int begin, int end) {        if (begin >= end) return 1;        int result = 0;        for (int i = begin; i <= end; i++)            result += helper(begin, i-1)*helper(i+1, end);        return result;    }        int numTrees(int n) {        return helper(1, n);        }};</span>