34 lines
758 B
C++
34 lines
758 B
C++
#include<iostream>
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#include<cstdio>
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#include<cstring>
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#include<cmath>
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#define eps 1e-7
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#define LL long long
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using namespace std;
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LL fac[21]={1};
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LL stir1[21][21];
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int main(){
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for(int i=1;i<21;i++)
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fac[i]=fac[i-1]*i;
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for(int i=1;i<=20;i++){
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stir1[i][0]=0;
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stir1[i][i]=1;
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for(int j=1;j<i;j++)
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stir1[i][j]=stir1[i-1][j-1]+(i-1)*stir1[i-1][j];
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}
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int t,n,k;
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scanf("%d",&t);
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while(t--){
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scanf("%d%d",&n,&k);
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if(n==1||k==0){
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printf("0.0000\n");
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continue;
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}
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LL sum=0;
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for(int i=1;i<=k;i++)
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sum+=stir1[n][i]-stir1[n-1][i-1];
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printf("%.4f\n",(double)sum/fac[n]);
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}
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return 0;
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}
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