CODEVS1993 草地排水

Posted 2020-09-01 ONION_CYC

【算法】网络流-最大流(dinic)

【题解】http://www.cnblogs.com/onioncyc/p/6496532.html

#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn=210,inf=0x3f3f3f3f;
struct edge{int from,v,flow;}e[maxn*3];
int n,m,tot=2,S,T,d[maxn],q[510],first[maxn],cur[maxn];
void insert(int u,int v,int flow)
{tot++;e[tot].v=v;e[tot].flow=flow;e[tot].from=first[u];first[u]=tot;}
bool bfs()
{
    memset(d,-1,sizeof(d));
    int head=0,tail=1;q[head]=S;
    d[S]=0;
    while(head!=tail)
     {
         int x=q[head++];if(head>=501)head=0;
         for(int i=first[x];i;i=e[i].from)
         if(d[e[i].v]==-1&&e[i].flow>0)
          {
              q[tail++]=e[i].v;d[e[i].v]=d[x]+1;
              if(tail>=501)tail=0;
          }
     }
    if(d[T]==-1)return 0;else return 1;
}
int dfs(int x,int a)
{
    if(x==T||a==0)return a;
    int flow=0,f;
    for(int& i=cur[x];i;i=e[i].from)//当前弧优化 
     if(d[e[i].v]==d[x]+1&&(f=dfs(e[i].v,min(a,e[i].flow)))>0)
      {
          e[i].flow-=f;
          e[i^1].flow+=f;
          flow+=f;
          a-=f;
          if(a==0)break;
      }
    return flow;
}
int main()
{
    scanf("%d%d",&m,&n);
    S=1;T=n;
    for(int i=1;i<=m;i++)
     {
         int u,v,w;
         scanf("%d%d%d",&u,&v,&w);
         insert(u,v,w);
         insert(v,u,0);
     }
    int ans=0;
    while(bfs())
     {
         for(int i=1;i<=n;i++)cur[i]=first[i];
         ans+=dfs(S,inf);
     }
    printf("%d",ans);
    return 0;
}

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