【BZOJ】1741: [Usaco2005 nov]Asteroids 穿越小行星群

#include<cstdio>
#include<algorithm>
#include<cstring> 
#include<queue>
using namespace std;
const int maxn=20010,inf=0x3f3f3f3f;//2*

int tot=1,first[maxn],cur[maxn],d[maxn],S,T,n,k;//tot=1
queue<int>q;
struct edge{int v,flow,from;}e[maxn*10];
void insert(int u,int v){
    tot++;e[tot].v=v;e[tot].flow=1;e[tot].from=first[u];first[u]=tot;
    tot++;e[tot].v=u;e[tot].flow=0;e[tot].from=first[v];first[v]=tot;
}
bool bfs(){
    memset(d,-1,sizeof(d));
    d[S]=0;q.push(S);
    while(!q.empty()){
        int x=q.front();q.pop();
        for(int i=first[x];i;i=e[i].from)if(e[i].flow&&d[e[i].v]==-1){
            d[e[i].v]=d[x]+1;
            q.push(e[i].v);
        }
    }
    return d[T]!=-1;
}
int dinic(int x,int a){
    if(x==T||a==0)return a;
    int flow=0,f;
    for(int &i=cur[x];i;i=e[i].from)
    if(e[i].flow&&d[e[i].v]==d[x]+1&&(f=dinic(e[i].v,min(a,e[i].flow)))>0){
        e[i].flow-=f;
        e[i^1].flow+=f;
        a-=f;
        flow+=f;
        if(a==0)break;
    }
    return flow;
}
int main(){
    scanf("%d%d",&n,&k);
    int u,v;
    S=0;T=n+n+1;
    for(int i=1;i<=k;i++){
        scanf("%d%d",&u,&v);
        insert(u,v+n);
    }
    for(int i=1;i<=n;i++){insert(S,i);insert(i+n,T);}
    int ans=0;
    while(bfs()){
        for(int i=S;i<=T;i++)cur[i]=first[i];//S-T
        ans+=dinic(S,inf);
    }
    printf("%d",ans);
    return 0;
}