Luogu2917_ [USACO08NOV]奶牛混合起来Mixed Up Cows_KEY

#include <cstdio>
using namespace std;

int N,K,a[17];
long long f[1<<16][17];

inline int abs(int x){return x>0?x:-x;}

int main()
{
    scanf("%d%d",&N,&K);
    register int i,j,k;
        for(i=1;i<=N;i++)scanf("%d",&a[i]),f[1<<i-1][i]=1ll;
        for(i=0;i<1<<N;i++){
            for(j=1;j<=N;j++){
                if(i&(1<<j-1)){
                    for(k=1;k<=N;k++){
                        if(!(i&(1<<k-1)) && abs(a[k]-a[j])>K){
                            f[i|(1<<k-1)][k]+=f[i][j];
                        }
                    }
                }
            }
        }
    long long ans=0;
        for(i=1;i<=N;i++)ans+=(long long)f[(1<<N)-1][i];
    printf("%lld",ans);
    return 0;
}