POJ 1149 PIGS

    https://vjudge.net/problem/POJ-1149

#include <iostream>
#include<queue> 
#include <cstring>
#define IOS std::ios::sync_with_stdio(0)
using namespace std;
 const int N =1000,M=5e5+100;
#define int long long 

 const int inf =1e9;
 int R[N],a[N];
 int all=1,hd[N],go[M],w[M],nxt[M];
 
 int S,T,n,m;
 int dis[M],ans=0,now[M];
 
 void add_(int x,int y,int z){
 	nxt[++all]=hd[x]; hd[x]=all; go[all]=y;
 	w[all]=z;
 	swap(x,y);
 	nxt[++all]=hd[x]; hd[x]=all; go[all]=y;
 	w[all]=0;
 }
  bool bfs(){
 	for(int i=1;i<=n+m+3;i++)dis[i]=inf;
 	queue<int> q;
 	q.push(S);
 	now[S]=hd[S];
 	dis[S]=0;
 	
 	while(q.empty()==0){
 		int x=q.front(); 
 		q.pop();
 		for(int i=hd[x];i;i=nxt[i]){
 			int y=go[i];
 			if(w[i]>0&&dis[y]==inf){
 				dis[y]=dis[x]+1;
 				now[y]=hd[y];
	 			q.push(y);
	 			if(y==T) return 1;
 			}
 		}
 	}
 	return 0;
 }
 int dfs(int x,int sum){
 	if(x==T) return sum;
 	int k,res=0;
 	
 	for(int i=now[x];i&∑i=nxt[i]){
 		now[x]=i;
 		int y=go[i]; 
 		if(w[i]>0&&(dis[y]==dis[x]+1)){
 			k=dfs(y,min(sum,w[i]));
 			if(k==0) dis[y]=inf;
 			w[i]-=k;
 			w[i^1]+=k;
 			res+=k;
 			sum-=k;
 		}
 	}
 	return res;
 }
 
 signed main(){
 	IOS;
 	int i,x,y,z;
 	cin>>n>>m>>S>>T;
 	for(i=1;i<=m;i++){
 		cin>>x>>y>>z; add_(x,y,z); 
 	}
 	int ans=0;
	while(bfs()) ans+=dfs(S,inf);
	cout<<ans<<endl;
 }