ZOJ.1938 Binomial Showdown【组合数】 2015/09/23

Binomial Showdown

---

Time Limit: 2 Seconds      Memory Limit: 65536 KB

---

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.

Sample Input

4 2
10 5
49 6
0 0

Sample Output

6
252
13983816

---

Source: University of Ulm Local Contest 1997

求C(n,k)，但是要注意是C(n,k)快还是C(n,n-k)快

#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;}