BZOJ 3925 [Zjoi2015]地震后的幻想乡 一道积分神题

https://www.lydsy.com/JudgeOnline/showsource.php?id=2689580

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
typedef double db;
const int N=10,M=46;
int x,y,n,m;
int sz[1<<N],link[1<<N];
db f[1<<N][M];
int main(){
    scanf("%d%d",&n,&m);
    for(register int i=0;i<m;++i){
        scanf("%d%d",&x,&y);
        --x,--y;
        link[x]|=(1<<y),link[y]|=(1<<x);
    }
    for(register int i=1;i<(1<<n);++i)
        sz[i]=sz[i>>1]+(i&1);
    for(register int S1=1;S1<(1<<n);++S1)
        if(S1&1){
            for(register int S2=(S1-1)&S1;S2;S2=(S2-1)&S1)
                if(S2&1){
                    int T=0;
                    for(register int i=0;i<n;++i)
                        if((S1>>i)&(~S2>>i)&1)
                            T+=sz[link[i]&S2];
                    for(register int i=0;i+T<=m;++i)
                        f[S1][i]+=1.00/(i+1+T)-f[S2][i+T];
                }
        }
    printf("%.6lf
",f[(1<<n)-1][0]);
    return 0;
}