【网络流#9】POJ 2135 Farm Tour 最小费用流 - 《挑战程序设计竞赛》例题

#include<cstdio>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<set>
#include<map>
#include<stack>
#include<vector>
#include<queue>
#include<string>
#include<sstream>
#define eps 1e-9
#define ALL(x) x.begin(),x.end()
#define INS(x) inserter(x,x.begin())
#define FOR(i,j,k) for(int i=j;i<=k;i++)
#define MAXN 1005
#define MAXM 40005
#define INF 0x3fffffff
using namespace std;
typedef long long LL;
int i,j,k,n,m,x,y,ans,big,cas,num,w,t,u,v,S,T;
bool flag;

int head[MAXN],vis[MAXN],dis[MAXN],pos[MAXN],Edge,size;
char s[305][305];
struct edgenode
{
    int to,next,w,cost;
} edge[MAXM];
 
void add_edge(int x,int y,int w,int cost)
{
    edge[Edge].to=y;
    edge[Edge].w=w;
    edge[Edge].cost=cost;
    edge[Edge].next=head[x];
    head[x]=Edge;
    Edge++;
     
     
    edge[Edge].to=x;
    edge[Edge].w=0;
    edge[Edge].cost=-cost;
    edge[Edge].next=head[y];
    head[y]=Edge;
    Edge++;
}
 
bool SPFA(int s, int t)
{
    int u,v,i;
    queue <int> q;
    memset(vis,0,sizeof(vis));
    for(i=0;i<size;i++) dis[i]=INF;
     dis[s]=0;
     vis[s]=1;
    q.push(s);
    while(!q.empty())
    {
        u=q.front(); q.pop(); vis[u]=0;
        for (i=head[u];i!=-1;i=edge[i].next)
          {
               v=edge[i].to;
               if(edge[i].w>0&&dis[u]+edge[i].cost<dis[v])
               {
                dis[v]=dis[u]+edge[i].cost;
                pos[v]=i;
                if(!vis[v])
                {
                     vis[v]=1;
                     q.push(v);
                }
               }
          }
    }
    return dis[t]!=INF;
}
int MinCostFlow(int s,int t)
{
    int i,cost=0,flow=0;
    while(SPFA(s,t))
    {
        int d=INF;
        for (i=t;i!=s;i=edge[pos[i]^1].to)
        {
            d=min(d,edge[pos[i]].w);
        }
        for(i=t;i!=s;i=edge[pos[i]^1].to)
        {
            edge[pos[i]].w-=d;
            edge[pos[i]^1].w+=d;
        }
        flow+=d;
        cost+=dis[t]*d;
    }
    return cost; // flow是最大流值
}


int main()
{
	memset(head,-1,sizeof(head));Edge=0;
	scanf("%d%d",&n,&m);
	for (i=1;i<=m;i++)
	{
		scanf("%d%d%d",&x,&y,&t); 
		add_edge(x,y,1,t);
		add_edge(y,x,1,t);
	}
	S=0;T=n+1;
	size=n+2;
	add_edge(S,1,2,0);
	add_edge(n,T,2,0);
	printf("%d\n",MinCostFlow(S,T));
	return 0;
}