#include<cmath>
#include<cstdio>
#include<cctype>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>
#define N 110
#define R register
using namespace std;

int n,q;
double g[N],f[N][N];

int main(){
  scanf("%d%d",&q,&n);
  if(q==1){
    g[1]=0.0;
    for(R int i=2;i<=n;++i) g[i]=g[i-1]+2.0/i;
    printf("%.6lf",g[n]);
  }
  else{
    for(R int i=1;i<=n;++i) f[i][0]=1.0;
    for(R int i=2;i<=n;++i)
      for(R int j=1;j<i;++j){
        for(R int k=1;k<i;++k)
          f[i][j]+=f[k][j-1]+f[i-k][j-1]-f[k][j-1]*f[i-k][j-1];
        f[i][j]/=(i-1);
      }
    double ans=0.0;
    for(R int i=1;i<=n;++i) ans+=f[n][i];
    printf("%.6lf\n",ans);
  }
  return 0;
}

题解

#include<cmath>
#include<cstdio>
#include<cctype>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>
#define N 110
#define R register
using namespace std;

int n,q;
double g[N],f[N][N];

int main(){
  scanf("%d%d",&q,&n);
  if(q==1){
    g[1]=0.0;
    for(R int i=2;i<=n;++i) g[i]=g[i-1]+2.0/i;
    printf("%.6lf",g[n]);
  }
  else{
    for(R int i=1;i<=n;++i) f[i][0]=1.0;
    for(R int i=2;i<=n;++i)
      for(R int j=1;j<i;++j){
        for(R int k=1;k<i;++k)
          f[i][j]+=f[k][j-1]+f[i-k][j-1]-f[k][j-1]*f[i-k][j-1];
        f[i][j]/=(i-1);
      }
    double ans=0.0;
    for(R int i=1;i<=n;++i) ans+=f[n][i];
    printf("%.6lf\n",ans);
  }
  return 0;
}

\(\color{#ad8b00}\texttt{I\ AK\ SSH}\)