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

class Solution {public:    int numTrees(int n) {        int res = 1;for (int i = 2; i <= n; i++){res = 2*(2*i-1)*res/(i+1);}return res;    }};