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

思路：递归，由于是二叉查找树，先选择任一结点根结点，假设为结点i，则[1，i-1]范围的结点为结点i的左子树结点，[i+1，n]范围的结点为结点i的右子树结点，则以结点i为根结点的BST个数为左，右子树可构成BST个数的乘积，基于这个思路，可以写出以下递归程序。

class Solution
{
public:
    int numTrees1(int start, int end)
    {
        if (start >= end)
        {
            return 1;
        }
        
        int totalNum = 0;
        for (int i = start; i <= end; i++)
        {
            totalNum += numTrees1(start, i-1) * numTrees1(i+1, end);
        }
        return totalNum;
    }

    int numTrees(int n)
    {
        return numTrees1(1, n);
    }
};