poj 2135 Farm Tour【 最小费用最大流 】

第一道费用流的题目---

其实---还是不是很懂，只知道沿着最短路找增广路

建图

源点到1连一条容量为2（因为要来回），费用为0的边

n到汇点连一条容量为2，费用为0的边

另外的就是题目中输入的了

另外这题是无向边-----

maxn 开到1000会re----

存个模板先吧------------------------------------------

#include<cstdio>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<set>
#include<map>
#include<stack>
#include<vector>
#include<queue>
#include<string>
using namespace std;

typedef long long LL;
const int maxn = 5000;
const int INF = (1 << 30) - 1;
 
int first[maxn],vis[maxn],dis[maxn],pos[maxn],ecnt,size;

struct Edge{
    int v,next,cap,cost;
} e[10*maxn];

void init(){
    ecnt = 0;
    memset(first,-1,sizeof(first));
}
  
void add_edge(int u,int v,int cap,int cost){
    e[ecnt].v = v;
    e[ecnt].cap = cap;
    e[ecnt].cost = cost;
    e[ecnt].next = first[u];
    first[u] = ecnt++;
    
    e[ecnt].v = u;
    e[ecnt].cap = 0;
    e[ecnt].cost = -cost;
    e[ecnt].next = first[v];
    first[v] = ecnt++;
}
  
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 = first[u]; ~i;i = e[i].next){
               v=e[i].v; 
               if(e[i].cap > 0&& dis[u]+e[i].cost < dis[v]){
                  dis[v]=dis[u]+e[i].cost;
                  pos[v]=i;
                  if(!vis[v]){
                     vis[v]=1;
                     q.push(v);
                  }
               }
          }
    }
    return dis[t] != INF;
}

LL MCMF(int s,int t)
{
    int i;
    LL cost=0,flow=0;
    while(SPFA(s,t)){
        int d=INF;
        for (i = t;i != s;i = e[pos[i]^1].v){
            d = min(d,e[pos[i]].cap);
        }
        for(i = t;i != s;i = e[pos[i]^1].v){
            e[pos[i]].cap -= d;
            e[pos[i]^1].cap += d;
        }
        flow += d;
        cost += dis[t]*d;
    }
    return cost;
}


int main(){
    int n,m;
    while(scanf("%d %d",&n,&m) != EOF){
        init();
        size = n+1;
        add_edge(0,1,2,0); add_edge(1,0,2,0);
        add_edge(n+1,n,2,0);add_edge(n,n+1,2,0);
        
        for(int i = 0;i < m;i++){
            int u,v,w;
            scanf("%d %d %d",&u,&v,&w);
            add_edge(u,v,1,w);
            add_edge(v,u,1,w);
        }
        printf("%I64d
",MCMF(0,n+1));
    }
    return 0;
}