BZOJ3444 最后的晚餐（并查集）

　　容易发现只要图中有非链部分则无解。剩下就非常简单了。

#include<iostream> 
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
int read()
{
    int x=0,f=1;char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}
    while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();
    return x*f;
}
#define N 500010
#define P 989381
int n,m,fa[N],degree[N],size[N];
struct data
{
    int x,y;
    bool operator <(const data&a) const
    {
        return x<a.x||x==a.x&&y<a.y;
    }
    bool operator ==(const data&a) const
    {
        return x==a.x&&y==a.y;
    }
}a[N];
int find(int x){return fa[x]==x?x:fa[x]=find(fa[x]);}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("bzoj3444.in","r",stdin);
    freopen("bzoj3444.out","w",stdout);
    const char LL[]="%I64d
";
#else
    const char LL[]="%lld
";
#endif
    n=read(),m=read();
    for (int i=1;i<=m;i++)
    {
        a[i].x=read(),a[i].y=read();
        if (a[i].x>a[i].y) swap(a[i].x,a[i].y);
    }
    sort(a+1,a+m+1);
    int t=unique(a+1,a+m+1)-a-1;
    for (int i=1;i<=n;i++) fa[i]=i;
    for (int i=1;i<=t;i++)
    if (find(a[i].x)==find(a[i].y)) {cout<<0;return 0;}
    else fa[find(a[i].x)]=find(a[i].y),degree[a[i].x]++,degree[a[i].y]++;
    int cnt=0;
    for (int i=1;i<=n;i++)
    if (degree[i]>2) {cout<<0;return 0;}
    else if (find(i)==i) cnt++;
    else size[find(i)]++;
    t=0;
    for (int i=1;i<=n;i++)
    if (find(i)==i&&size[i]) t++;
    int ans=1;
    for (int i=1;i<=t;i++) ans=(ans<<1)%P;
    for (int i=1;i<=cnt;i++) ans=1ll*ans*i%P;
    cout<<ans;
    return 0;
}