奶牛的电信——网络流——dinic

#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector> 
#include<queue>
using namespace std;
inline int read(){
    int t=1,num=0;
    char c=getchar();
    while(c>'9'||c<'0'){if(c=='-')t=-1;c=getchar();}
    while(c>='0'&&c<='9'){num=num*10+c-'0';c=getchar();}
    return num*t;
}
const int INF=10000010;
struct edge{
    int to;int c;int rev;
};
vector<edge> g[401];
int n,m,iter[401],level[401];
void add(int f,int t,int c){
    g[f].push_back((edge){t,c,g[t].size()});
    g[t].push_back((edge){f,0,g[f].size()-1});
}
void bfs(int s){
    memset(level,0,sizeof(level));
    queue<int>q;
    level[s]=1;
    q.push(s);
    while(!q.empty()){
        int v=q.front();q.pop();
        for(int i=0;i<g[v].size();i++){
            edge &e=g[v][i];
            if(e.c>0&&level[e.to]==0){
                level[e.to]=level[v]+1;
                q.push(e.to);
            }
        }
    }
}
int dfs(int v,int t,int f){
    if(v==t) return f;
    for(int &i=iter[v];i<g[v].size();i++){
        edge &e=g[v][i];
        if(e.c>0&&level[v]+1==level[e.to]){
            int d=dfs(e.to,t,min(f,e.c));
            if(d>0){
                e.c-=d;
                g[e.to][e.rev].c+=d;
                return d;
            }
        }
    }
    return 0;
}
int flow(int s,int t){
    int flow=0;
    while(1){
        bfs(s);
        if(level[t]==0) return flow;
        memset(iter,0,sizeof(iter));
        int f;
        while((f=dfs(s,t,INF))>0)flow+=f;
    }
}
int main()
{
    int s,t;
    n=read();m=read();s=read()+n;t=read();
    for(int i=1;i<=n;i++)add(i,i+n,1);
    int x,y;
    while(m--){
        x=read();y=read();
        add(x+n,y,INF);
        add(y+n,x,INF);
    }
    printf("%d\n",flow(s,t));
    return 0;
}

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