以前刷试炼场时根本不会的
现在 一眼最小割！
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#define MAXN 1000000
#define INF 100000000
using namespace std;
int n,m,s,t;
int tot=2,g[MAXN],num[MAXN],nnext[MAXN],flow[MAXN];
int team[MAXN],head,tail,d[MAXN];
void Add(int x,int y,int z){nnext[tot]=g[x];g[x]=tot;num[tot]=y;flow[tot]=z;tot++;}
bool bfs()
{head=tail=0;memset(d,0,sizeof(d));d[s]=1;team[++tail]=s;while(head<tail){int x=team[++head];for(int i=g[x];i;i=nnext[i])if(d[num[i]]==0&&flow[i]!=0)d[num[i]]=d[x]+1,team[++tail]=num[i];}if(d[t]==0) return false;return true;
}
int dfs(int x,int mmin)
{if(x==t) return mmin;int tmp,f=0;for(int i=g[x];i;i=nnext[i])if(d[num[i]]==d[x]+1&&flow[i]&&(tmp=dfs(num[i],min(mmin,flow[i])))){flow[i]-=tmp,flow[i^1]+=tmp;f+=tmp,mmin-=tmp;if(mmin==0) return f;}return f;
}
int main()
{scanf("%d %d %d %d",&n,&m,&s,&t);for(int x,y,i=1;i<=m;i++){scanf("%d %d",&x,&y);Add(x+n,y,INF),Add(y,x+n,0);Add(y+n,x,INF),Add(x,y+n,0);}for(int i=1;i<=n;i++) Add(i,i+n,1),Add(i+n,i,0);int ans=0;while(bfs()) ans+=dfs(s+n,INF);printf("%d\n",ans);return 0;
}
