HDU 2426 Interesting Housing Problem 最小费用流/KM

http://acm.hdu.edu.cn/showproblem.php?pid=2426

分宿舍，有一句

 Note that you can never assign a student to a room which he/she has not rated, as the absence of rating indicates that the student cannot live in the room for other reasons

负数的不能分配吗?就WA在这里，在研究研究

代码：

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#define nMAX 5005
#define mMAX 500005
#define inf 999999999
using namespace std;
struct Edge
{
    int u,v,cap,cost,nxt;
}edge[mMAX];
int head[nMAX],dis[nMAX],pre[nMAX],qu[nMAX];
int s_edge,ans,maxf;
bool vs[nMAX];
void addedge(int u,int v,int ca,int co)
{
    edge[++s_edge].v=v;
    edge[s_edge].cap=ca;
    edge[s_edge].cost=co;
    edge[s_edge].nxt=head[u];
    head[u]=s_edge;

    edge[++s_edge].v=u;
    edge[s_edge].cap=0;
    edge[s_edge].cost=-co;
    edge[s_edge].nxt=head[v];
    head[v]=s_edge;
}
bool spfa(int s,int t,int n)//起点 终点 总数
{
    int start=0,tail=1;
    for(int i=0;i<=n;i++)
    {
        dis[i]=-inf;
        vs[i]=0;
    }
    qu[0]=s;
    dis[s]=0;//是吗???
    vs[s]=1;
    while(start!=tail)
    {
        int u=qu[start];
        for(int e=head[u];e;e=edge[e].nxt)
        {
           int v=edge[e].v;
           if(edge[e].cap&&dis[v]<dis[u]+edge[e].cost)
           {
               dis[v]=dis[u]+edge[e].cost;
               pre[v]=e;
               if(!vs[v])
               {
                   vs[v]=1;
                   qu[tail++]=v;
                   if(tail==nMAX)tail=0;
               }
           }
        }
        vs[u]=0;
        start++;
        if(start==nMAX)start=0;
    }
    if(dis[t]==-inf)return 0;
    return 1;
}

void end(int s,int t)
{
    int u,p;
    for(u=t;u!=s;u=edge[p^1].v)
    {
        p=pre[u];
        edge[p].cap-=1;
        edge[p^1].cap+=1;
        ans+=edge[p].cost;
    }
    maxf+=1;
}

int main()
{
    int i,j,k,s,t,N,M,E,CASE=0;
    while(~scanf("%d%d%d",&N,&M,&E))
    {
        CASE++;
        s=N+M,t=N+M+1;//!!!
        memset(head,0,sizeof(head));
        s_edge=1;
        for(i=0;i<N;i++)
          addedge(s,i,1,0);
        for(i=0;i<M;i++)
          addedge(i+N,t,1,0);
        while(E--)
        {
            scanf("%d%d%d",&i,&j,&k);
            if(k<0)continue;
            addedge(i,j+N,1,k);
        }
        ans=0, maxf=0;
        while(spfa(s,t,N+M+2))
        {
            end(s,t);
        }
        if(maxf==N)printf("Case %d: %d\n",CASE,ans);
        else printf("Case %d: -1\n",CASE);
    }
    return 0;
}