模板—网络流

模板—网络流

 Code:

#include <queue>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define N 100010
#define M 1000010
#define inf 1000000000
int head[N],cur[N],to[M<<1],val[M<<1],nxt[M<<1],idx=1,ans,dis[N],n,m,s,t;
void add(int a,int b,int c)
	{nxt[++idx]=head[a],to[idx]=b,val[idx]=c,head[a]=idx;}
bool bfs()
{
	memset(dis,-1,sizeof dis);
	queue <int> q; q.push(s),dis[s]=0;
	while(!q.empty())
	{
		int p=q.front(); q.pop();
		for(int i=head[p];i;i=nxt[i])
			if(dis[to[i]]==-1&&val[i])
				dis[to[i]]=dis[p]+1,q.push(to[i]);
	} return dis[t]!=-1;
}
int dfs(int p,int flow)
{
	int temp=flow,now;
	if(p==t) return flow;
	for(int i=cur[p];i;i=nxt[i])
		if(val[i]&&dis[to[i]]==dis[p]+1)
		{
			now=dfs(to[i],min(temp,val[i]));
			if(!now) dis[to[i]]==-1;
			val[i]-=now,val[i^1]+=now,temp-=now;
			if(val[i]) cur[p]=i;
			if(!temp) break;
		} return flow-temp;
}
void dinic() {while(bfs()) memcpy(cur,head,sizeof cur),ans+=dfs(s,inf);}
int main()
{
	scanf("%d%d%d%d",&n,&m,&s,&t);
	for(int i=1,x,y,z;i<=m;i++) scanf("%d%d%d",&x,&y,&z),add(x,y,z),add(y,x,0);
	dinic(),printf("%d
",ans);
}