BZOJ2768 JLOI2012冠军调查（最小割）

　　容易想到网络流。将每个人拆成0和1两个点。若某人值为0的话则让源连向0，否则让1连向汇，流量为1。相互认识的人之间01各自连边。跑最小割即可。

#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 310 
#define S 0
#define T 601
int n,m,p[N<<1],t=-1,ans=0;
int d[N<<1],cur[N<<1],q[N<<1];
struct data{int to,nxt,cap,flow;
}edge[N*N<<1];
int trans(int x,int y){return (x<<1)-y;}
void addedge(int x,int y)
{
    t++;edge[t].to=y,edge[t].nxt=p[x],edge[t].cap=1,edge[t].flow=0,p[x]=t;
    t++;edge[t].to=x,edge[t].nxt=p[y],edge[t].cap=0,edge[t].flow=0,p[y]=t;
}
bool bfs()
{
    memset(d,255,sizeof(d));d[S]=0;
    int head=0,tail=1;q[1]=S;
    do
    {
        int x=q[++head];
        for (int i=p[x];~i;i=edge[i].nxt)
        if (d[edge[i].to]==-1&&edge[i].flow<edge[i].cap)
        {
            d[edge[i].to]=d[x]+1;
            q[++tail]=edge[i].to;
        }
    }while (head<tail);
    return ~d[T];
}
int work(int k,int f)
{
    if (k==T) return f;
    int used=0;
    for (int i=cur[k];~i;i=edge[i].nxt)
    if (d[k]+1==d[edge[i].to])
    {
        int w=work(edge[i].to,min(f-used,edge[i].cap-edge[i].flow));
        edge[i].flow+=w,edge[i^1].flow-=w;
        if (edge[i].flow<edge[i].cap) cur[k]=i;
        used+=w;if (used==f) return f;
    }
    if (used==0) d[k]=-1;
    return used;
}
void dinic()
{
    while (bfs())
    {
        memcpy(cur,p,sizeof(p));
        ans+=work(S,N*N);
    }
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("bzoj2768.in","r",stdin);
    freopen("bzoj2768.out","w",stdout);
    const char LL[]="%I64d
";
#else
    const char LL[]="%lld
";
#endif
    n=read(),m=read();
    memset(p,255,sizeof(p));
    for (int i=1;i<=n;i++)
    {
        int x=read();
        if (x) addedge(trans(i,1),T);
        else addedge(S,trans(i,0));
    }
    for (int i=1;i<=m;i++)
    {
        int x=read(),y=read();
        addedge(trans(x,0),trans(y,1));
        addedge(trans(y,0),trans(x,1));
    }
    dinic();
    cout<<ans;
    return 0;
}