这里有一篇很好很强的博客%%%
YouSiki大佬的博客
多理解一会就行了…
代码
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int MAXN = 10;
const int MAXM = 46;
const int MAXS = 1<<10;
int n, m, e[MAXN], bitcnt[MAXS], cnt[MAXS];
LL f[MAXS][MAXM], g[MAXS][MAXM], c[MAXM][MAXM];
inline void Pre() {
c[0][0] = 1;
for(int i = 1; i < MAXM; ++i) {
c[i][0] = c[i][i] = 1;
for(int j = 1; j < i; ++j)
c[i][j] = c[i-1][j-1] + c[i-1][j];
}
}
int main () {
Pre();
scanf("%d%d", &n, &m);
for(int i = 0, x, y; i < m; ++i)
scanf("%d%d", &x, &y), --x, --y, e[x] |= 1<<y, e[y] |= 1<<x;
for(int s = 1; s < (1<<n); ++s) {
if((bitcnt[s] = bitcnt[s>>1] + (s&1)) == 1) { g[s][0] = 1; continue; }
for(int i = 0; i <= n; ++i)
if(s&(1<<i)) cnt[s] += bitcnt[e[i]&s];
cnt[s]>>=1;
int one = s&-s, outside = s^one;
for(int sub = outside; sub; sub = (sub-1)&outside)
for(int i = 0; i <= cnt[sub]; ++i)
for(int j = 0; j <= cnt[s^sub]; ++j)
f[s][i+j] += c[cnt[sub]][i] * g[s^sub][j];
for(int i = 0; i <= cnt[s]; ++i) g[s][i] = c[cnt[s]][i] - f[s][i];
}
double ans = 0;
for(int i = 0; i < m; ++i)
ans += (double)f[(1<<n)-1][i]/c[cnt[(1<<n)-1]][i]; //注意转型为double
printf("%.6f
", ans/(m+1));
}