Leetcode:Unique Binary Search Trees

class Solution {public:     long long  f(int n, int m)    {        long long  result;        result=1;        for(int i=n;i<=m;i++)        {            result=result*i;        }        return result;            }    int numTrees(int n) {//卡特兰数的运用，C_n=(2n)!/((n+1)!*n!),但考虑到溢出问题，需要化简后计算        int result;        result=f(n+2,2*n)/f(1,n);        return result;    }};