poj 2135--Farm Tour--cost flow

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <vector> 
#include <queue>
using namespace std;
const int N=1e3+10;
const int inf=0x3f3f3f3f;
struct Edge
{
    
    
    int from,to,cap,flow,w;
};
vector<Edge>edges;
vector<int>G[N];
void add(int from,int to,int cap,int w)
{
    
    
    edges.push_back((Edge){
    
    from,to,cap,0,w});
    edges.push_back((Edge){
    
    to,from,0,0,-w});
    int m=edges.size();
    G[from].push_back(m-2);
    G[to].push_back(m-1);
}

int n,m,preV[N],preE[N],dis[N];
bool vis[N];
bool spfa(int s,int t){
    
    
	memset(preV,-1,sizeof preV);
	memset(vis,false,sizeof vis);
	memset(dis,inf,sizeof dis);
	queue<int>que;
	que.push(s);
	vis[s]=true;
	dis[s]=0;
	while(!que.empty()){
    
    
		int u=que.front();
		que.pop();
		vis[u]=false;
		for(int i=0;i<G[u].size();++i){
    
    
			Edge &e=edges[G[u][i]];
			
			if(e.cap-e.flow>0){
    
    
				
				int v=e.to;
				if(dis[u]+e.w<dis[v]){
    
    
					dis[v]=dis[u]+e.w;
					preV[v]=u;
					preE[v]=i;
					if(!vis[v]){
    
    
						que.push(v);
						vis[v]=true;
					}
				}
			}
		}
	}
	return dis[t]!=inf;
}
int mincost(int s,int t){
    
    
	int	mincost=0,maxflow=0;
	while(spfa(s,t)){
    
    
		int v=t,flow=inf;
		while(preV[v]!=-1){
    
    
			int u=preV[v],i=preE[v];
			Edge &e=edges[G[u][i]];
			flow=min(flow,e.cap-e.flow);
			v=u;
		}
		v=t;
		while(preV[v]!=-1){
    
    
			int u=preV[v],i=preE[v];
			Edge &e=edges[G[u][i]];
			e.flow+=flow;
			Edge &e1=edges[G[u][i]^1];
			e1.flow-=flow;
			v=u;
		};
		mincost+=dis[t]*flow;
		maxflow+=flow;
	}
	return mincost;
}
void init(){
    
    
	edges.clear();
	for(int i=0;i<=n+2;++i) G[i].clear();
}
int main(){
    
    
	while(~scanf("%d %d",&n,&m)){
    
    
		init();
		for(int i=1;i<=m;++i){
    
    
			int u,v,w;
			scanf("%d %d %d",&u,&v,&w);
			add(u,v,1,w);
			add(v,u,1,w);
		}
		int s=0,t=n+1;
		add(s,1,2,0);
		add(n,t,2,0);
		printf("%d\n",mincost(s,t));
	}
	return 0;
}