UVA 10375 Choose and divide

The binomial coefficient C(m, n) is defined as
C(m, n) = m!
(m − n)! n!
Given four natural numbers p, q, r, and s, compute the the result of dividing C(p, q) by C(r, s).
Input
Input consists of a sequence of lines. Each line contains four non-negative integer numbers giving values
for p, q, r, and s, respectively, separated by a single space. All the numbers will be smaller than 10,000
with p ≥ q and r ≥ s.
Output
For each line of input, print a single line containing a real number with 5 digits of precision in the fraction,
giving the number as described above. You may assume the result is not greater than 100,000,000.
Sample Input
10 5 14 9
93 45 84 59
145 95 143 92
995 487 996 488
2000 1000 1999 999
9998 4999 9996 4998
Sample Output
0.12587
505606.46055
1.28223
0.48996
2.00000
3.99960

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用唯一分解定理，记录次数，最后求出答案。

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#include<iostream>#include<cstring>#include<cstdio>#include<cmath>using namespace std;const int N=10005;int p,q,r,s,cnt,pri[N],c[N];bool vis[N];double ans;void add(int x,int y){    for(int i=1;i<=x;i++)    {        int tmp=i;        for(int j=1;j<=cnt&&y!=0;j++)            while(tmp%pri[j]==0)            {                tmp/=pri[j];                c[j]+=y;            }    }}void euler(){    vis[1]=1;    for(int i=2;i<=10000;i++)    {        if(!vis[i])            pri[++cnt]=i;        for(int j=1;j<=cnt&&pri[j]*i<=10000;j++)        {            vis[i*pri[j]]=1;            if(i%pri[j]==0)                break;        }    }}int main(){    euler();    while(scanf("%d%d%d%d",&p,&q,&r,&s)==4)    {        ans=1;        memset(c,0,sizeof(c));        add(p,1);        add(p-q,-1);        add(q,-1);        add(r,-1);        add(r-s,1);        add(s,1);        for(int i=1;i<=cnt;i++)            ans*=pow(pri[i],c[i]);        printf("%.5f\n",ans);    }    return 0;}