POJ 2249 Binomial Showdown（排列组合）

Description
In how many ways can you choose k elements out of n elements, not taking order into account?
Write a program to compute this number.
Input
The input will contain one or more test cases.
Each test case consists of one line containing two integers n (n>=1) and k (0<=k<=n).
Input is terminated by two zeroes for n and k.
Output
For each test case, print one line containing the required number. This number will always fit into an integer, i.e. it will be less than 2 31.
Warning: Don't underestimate the problem. The result will fit into an integer - but if all intermediate results arising during the computation will also fit into an integer depends on your algorithm. The test cases will go to the limit.
Sample Input
4 2
10 5
49 6
0 0
Sample Output
6
252

13983816

#include<iostream>#include<cstdio>#include<cstring>using namespace std;__int64 f[10000000];int main(){    __int64 n,m;    while(scanf("%I64d%I64d",&n,&m),n||m)    {        f[0]=1;        if(m>n/2)///优化不然会超时            m=n-m;        for(int i=1; i<=m; i++)        {            f[i]=f[i-1]*(n-i+1)/i;        }        printf("%I64d\n",f[m]);    }}