记录详情: https://www.luogu.com.cn/record/63156415

代码:

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int M=5010;

struct node{
	int to,next,dis;
}edge[M<<1];
int head[M],cnte;
void add(int u,int v,int w){
	edge[++cnte].to=v;
	edge[cnte].dis=w;
	edge[cnte].next=head[u];
	head[u]=cnte;
}

int n,m,s,t,ans;

int dis[M];
bool bfs(){
	for(int i=0;i<M;i++) dis[i]=0;
	dis[s]=1;
	queue<int>q;
	q.push(s);
	while(!q.empty()){
		int u=q.front();q.pop();
		for(int i=head[u];~i;i=edge[i].next){
			int v=edge[i].to;
			if(!dis[v]&&edge[i].dis>0) dis[v]=dis[u]+1,q.push(v);
		}
	}
	return dis[t];
}

int dfs(int u,int in){
	if(u==t) return in;
	int out=0;
	for(int i=head[u];~i&&in>0;i=edge[i].next){
		int v=edge[i].to;
		if(edge[i].dis>0&&dis[v]==dis[u]+1){
			int res=dfs(v,min(edge[i].dis,in));
			edge[i].dis-=res;
			edge[i^1].dis+=res;
			in-=res,out+=res;
		}
	}
	if(out==0) dis[u]=-100000;
	return out;
}

signed main(){
	for(int i=0;i<M;i++) head[i]=-1;
	scanf("%lld%lld%lld%lld",&n,&m,&s,&t);
	for(int i=1,u,v,w;i<=m;i++){
		scanf("%lld%lld%lld",&u,&v,&w);
		add(u,v,w),add(v,u,0);
	}
	while(bfs()) ans+=dfs(s,1e18);
	printf("%lld",ans);
	return 0;
}