COGS 13. 运输问题4

#include <cstdio>
#include <queue>
#define inf 0x7fffffff
#define N 105

using namespace std;
bool vis[N];
int n,s,t,fa[N],dis[N],flow[N],cnt=1,head[N];
struct Edge
{
    int next,to,flow,cost;
    Edge (int next=0,int to=0,int flow=0,int cost=0) : next(next),to(to),flow(flow),cost(cost) {}
}edge[N*N];
inline void ins(int u,int v,int w,int l)
{
    edge[++cnt]=Edge(head[u],v,w,l);
    head[u]=cnt;
}
bool spfa(int s,int t)
{
    for(int i=s;i<=t;++i) vis[i]=0,flow[i]=inf,dis[i]=inf;
    dis[s]=0;
    fa[s]=0;
    queue<int>q;
    q.push(s);
    for(int now;!q.empty();)
    {
        now=q.front();
        q.pop();
        vis[now]=0;
        for(int i=head[now];i;i=edge[i].next)
        {
            int v=edge[i].to;
            if(dis[v]>dis[now]+edge[i].cost&&edge[i].flow)
            {
                dis[v]=dis[now]+edge[i].cost;
                flow[v]=min(flow[now],edge[i].flow);
                fa[v]=i;
                if(!vis[v])
                {
                    q.push(v);
                    vis[v]=1; 
                }
            }
        }
    }
    return dis[t]!=inf;
}
int dinic(int s,int t)
{
    int ans=0;
    for(;spfa(s,t);)
    {
        int x=flow[t];
        for(int i=t;i!=s&&i;i=edge[fa[i]^1].to)
        {
            edge[fa[i]].flow-=x;
            edge[fa[i]^1].flow+=x;
        }
        ans+=dis[t]*x;
    }
    return ans;
}
int main()
{
    freopen("maxflowd.in","r",stdin);freopen("maxflowd.out","w",stdout);
    scanf("%d%d%d",&n,&s,&t);
    for(int i=1;i<=n;++i)
     for(int a,b,j=1;j<=n;++j)
     {
         scanf("%d%d",&a,&b);
         if(a&&b)
         {
             ins(i,j,a,b);
             ins(j,i,0,-b);
        }
     }
    printf("%d\n",dinic(s,t));
    return 0;
}