Luo Gu P1345 [USACO5.4] cows telecommunications Telecowmunication

• 题意: minimum percentage
• Ideas: the right to point 1, the right side of the infinite, we should point out to dismantle point and point, as specified by the source and sink and sink point source can not be cut off, even to the point of super-source source point, super sinks to the point of even sinks.

#include<bits/stdc++.h>
#define ll long long
using namespace std;
typedef  pair<int,int> pii;
const int N = 2000;
const int INF = 0x3f3f3f3f;

struct E{
    int u,v,flow,nxt;
    E(){}
    E(int u,int v,int flow,int nxt):u(u),v(v),flow(flow),nxt(nxt){}
}e[N*2];

int n,m,sp,tp,tot;
int head[N],dis[N],pre[N],cur[N];
void init(){
    tot = 0;    memset(head,-1,sizeof head);
}
void addE(int u,int v,int flow){
    e[tot].u = u; e[tot].v = v; e[tot].flow = flow; e[tot].nxt = head[u]; head[u] = tot++;
    e[tot].u = v; e[tot].v = u; e[tot].flow = 0; e[tot].nxt = head[v]; head[v] = tot++;
}
int q[N];
int bfs(){
    int qtop = 0,qend=0;
    memset(dis,-1,sizeof dis);  
    dis[sp] = 0;    
    q[qend++] = sp;
    while(qtop!=qend){
        int u = q[qtop++];
        if(u==tp)   return true;
        for(int i=head[u];~i;i=e[i].nxt){
            int v = e[i].v;
            if(dis[v]==-1 && e[i].flow){
                dis[v] = dis[u]+1;  
                q[qend++] = v;
            }
        }
    }
    return dis[tp]!=-1;
}
int dfs(int u,int flow){
    int res = 0;
    if(u==tp)   return flow;
    for(int i=head[u];i!=-1&&flow;i=e[i].nxt){
        int v = e[i].v;
        if(dis[v]==dis[u]+1 && e[i].flow){
            int d = dfs(v,min(e[i].flow,flow));
            e[i].flow -=d;
            e[i^1].flow += d;
            res+=d;
            flow -= d;
        }
    }
    if(!res)
        dis[u] = -2;
    return res;
}
int dinic(){
    int ans=0;
    while(bfs()){
        ans+=dfs(sp,INF);
    }
    return ans;
}
int main(){
    int c1,c2;
    scanf("%d%d%d%d",&n,&m,&c1,&c2);
    sp = n*2 +1 , tp = sp +1;
    int u,v;
    init();
    addE(sp,c1+n,INF);
    addE(c2,tp,INF);

    for(int i=1;i<=n;++i)   addE(i,i+n,1);
    for(int i=1;i<=m;++i){
        scanf("%d%d",&u,&v);
        addE(u+n,v,INF);
        addE(v+n,u,INF);
    }

    printf("%d\n",dinic());
    return 0;
}