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.

去网上搜n个二叉搜索树的递推公式或者Catalan数，可以由h(n)=C(2n,n)/(n+1)得到h(n)=[2n*(2n-1)*...(n+2)]/[n!].

class Solution {public:    int numTrees(int n) {        long long facN=1,fac2N=1;        for(long long i=1;i<=n;i++)            facN*=i;        for(long long i=n+2;i<=2*n;i++)            fac2N*=i;        long long res=fac2N/facN;        return res;    }};