codeforce 906 SOL （C）

我们发现N很小，那么我们可以爆搜每个人取或不取，最后统计答案。

#include<bits/stdc++.h>
#define eho(x) for(int i=head[x];i;i=net[i])
#define N 27
#define M 4007
using namespace std;
int anw,ans[N],now,nows[N],siz,n,m;
int f[N][N],fall[M],net[M],head[N],tot,a,b;
#define sight(c) ('0'<=c&&c<='9')
inline void read(int &x){
    static char c;
    for (c=getchar();!sight(c);c=getchar());
    for (x=0;sight(c);c=getchar())x=x*10+c-48;
}
void write(int x){if (x<10) {putchar('0'+x); return;} write(x/10); putchar('0'+x%10);}
inline void writeln(int x){ if (x<0) putchar('-'),x*=-1; write(x); putchar('\n'); }
inline void writel(int x){ if (x<0) putchar('-'),x*=-1; write(x); putchar(' '); }
void dfs(int x){
    if (now>=anw) return;
    if (n==x) {
        for (int i=0;i<n;i++) 
         if (f[n][i]^siz) return;
        memcpy(ans,nows,sizeof nows);
        anw=now; return;
    }
    memcpy(f[x+1],f[x],sizeof f[x]);
    dfs(x+1);
    for (int i=0;i<n;i++)
     if(f[x][x]&(1<<(i)))
       f[x+1][i]|=f[x][x]; 
    nows[++now]=x+1; dfs(x+1); now--;
}
int main () {
    freopen("c.in","r",stdin);
    read(n); read(m);
    for (int i=0;i<n;i++)ans[i]=i+1,f[0][i]|=1<<i;anw=n;
    siz=1<<n; siz--;
    while (m--) {
        read(a); read(b);
        f[0][a-1]|=1<<b-1;f[0][b-1]|=1<<a-1;
    }
    dfs(0);
    writeln(anw);
    for (int i=1;i<=anw;i++)
    writel(ans[i]);return 0;
}