POJ2112 Optimal Milking（最大流）

先Floyd求牛到机器最短距离，然后二分枚举最长的边。

 #include<cstdio>
 #include<cstring>
 #include<queue>
 #include<algorithm>
 using namespace std;
 #define INF (1<<30)
 #define MAXN 233
 #define MAXM 233*233*20 

 struct Edge{
     int v,cap,flow,next;
 }edge[MAXM];
 int vs,vt,NE,NV;
 int head[MAXN];

 void addEdge(int u,int v,int cap){
     edge[NE].v=v; edge[NE].cap=cap; edge[NE].flow=;
     edge[NE].next=head[u]; head[u]=NE++;
     edge[NE].v=u; edge[NE].cap=; edge[NE].flow=;
     edge[NE].next=head[v]; head[v]=NE++;
 }

 int level[MAXN];
 int gap[MAXN];
 void bfs(){
     memset(level,-,sizeof(level));
     memset(gap,,sizeof(gap));
     level[vt]=;
     gap[level[vt]]++;
     queue<int> que;
     que.push(vt);
     while(!que.empty()){
         int u=que.front(); que.pop();
         for(int i=head[u]; i!=-; i=edge[i].next){
             int v=edge[i].v;
             if(level[v]!=-) continue;
             level[v]=level[u]+;
             gap[level[v]]++;
             que.push(v);
         }
     }
 }

 int pre[MAXN];
 int cur[MAXN];
 int ISAP(){
     bfs();
     memset(pre,-,sizeof(pre));
     memcpy(cur,head,sizeof(head));
     int u=pre[vs]=vs,flow=,aug=INF;
     gap[]=NV;
     while(level[vs]<NV){
         bool flag=false;
         for(int &i=cur[u]; i!=-; i=edge[i].next){
             int v=edge[i].v;
             if(edge[i].cap!=edge[i].flow && level[u]==level[v]+){
                 flag=true;
                 pre[v]=u;
                 u=v;
                 //aug=(aug==-1?edge[i].cap:min(aug,edge[i].cap));
                 aug=min(aug,edge[i].cap-edge[i].flow);
                 if(v==vt){
                     flow+=aug;
                     for(u=pre[v]; v!=vs; v=u,u=pre[u]){
                         edge[cur[u]].flow+=aug;
                         edge[cur[u]^].flow-=aug;
                     }
                     //aug=-1;
                     aug=INF;
                 }
                 break;
             }
         }
         if(flag) continue;
         int minlevel=NV;
         for(int i=head[u]; i!=-; i=edge[i].next){
             int v=edge[i].v;
             if(edge[i].cap!=edge[i].flow && level[v]<minlevel){
                 minlevel=level[v];
                 cur[u]=i;
             }
         }
         if(--gap[level[u]]==) break;
         level[u]=minlevel+;
         gap[level[u]]++;
         u=pre[u];
     }
     return flow;
 }

 int d[MAXN][MAXN],k,c,m,N;
 bool available(int mx){
     memset(head,-,sizeof(head));
     for(int i=; i<=k; ++i) addEdge(i,vt,m);
     for(int i=; i<=c; ++i) addEdge(vs,i+k,);
     for(int i=; i<=c; ++i){
         for(int j=; j<=k; ++j){
             if(d[i+k][j]== || d[i+k][j]>mx) continue;
             addEdge(i+k,j,);
         }
     }
     return ISAP()==c;
 }
 inline void floyd(){
     for(int k=; k<=N; ++k){
         for(int i=; i<=N; ++i){
             if(!d[i][k]) continue;
             for(int j=; j<=N; ++j){
                     if(d[k][j] && (d[i][j]== || d[i][j]>d[i][k]+d[k][j])) d[i][j]=d[i][k]+d[k][j];
             }
         }
     }
 }
 int main(){
     scanf("%d%d%d",&k,&c,&m);
     N=k+c;

     vs=; vt=N+; NV=vt+;

     for(int i=; i<=N; ++i){
         for(int j=; j<=N; ++j) scanf("%d",&d[i][j]);
     }
     floyd();

     int l=,r=INF;
     while(l<r){
         int mid=l+r>>;
         if(available(mid)) r=mid;
         else l=mid+;
     }
     printf("%d",l);
     return ;
 }