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 = ;
 const int INF = ( << ) - ;

 int first[maxn],vis[maxn],dis[maxn],pos[maxn],ecnt,size;

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

 void init(){
     ecnt = ;
     memset(first,-,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 = ;
     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,,sizeof(vis));
     for(i= ;i <= size;i++) dis[i]=INF;

     dis[s]=;
     vis[s]=;
     q.push(s);

     while(!q.empty()){
         u=q.front(); q.pop(); vis[u]=;
         for (i = first[u]; ~i;i = e[i].next){
                v=e[i].v;
                if(e[i].cap > && dis[u]+e[i].cost < dis[v]){
                   dis[v]=dis[u]+e[i].cost;
                   pos[v]=i;
                   if(!vis[v]){
                      vis[v]=;
                      q.push(v);
                   }
                }
           }
     }
     return dis[t] != INF;
 }

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

         for(int i = ;i < m;i++){
             int u,v,w;
             scanf("%d %d %d",&u,&v,&w);
             add_edge(u,v,,w);
             add_edge(v,u,,w);
         }
         printf("%I64d\n",MCMF(,n+));
     }
     return ;
 }