Poj.2249 Binomial Showdown【组合数】 2015/09/23

Binomial Showdown

Time Limit: 1000MS Memory Limit: 65536KTotal Submissions: 18695 Accepted: 5713

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 2³¹. 
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

4 210 549 60 0

Sample Output

625213983816

Source

Ulm Local 1997

同ZOJ1938

#include<iostream>#include<cstdio>#include<cstring>#include<cmath>using namespace std;int min(int a,int b){    return a>b?b:a;}long long cal(int n,int k){    long long ret = 1;    k = min(k,n-k);    for( int i = 0 ; i < k ; ++i )        ret = (ret * (n-i) / (i+1) );//    for( int i = 2 ; i <= k ; ++i )//        ret /= i;    return ret;}int main(){    int n,k;    while( ~scanf("%d %d",&n,&k) ){        if( !n&&!k ) break;        printf("%lld\n",cal(n,k));    }    return 0;}