【最小割】【Dinic】Gym

http://blog.csdn.net/lxy767087094/article/details/68942422

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
using namespace std;
const int dx[]={0,-1,0,1},dy[]={-1,0,1,0};
typedef long long ll;
const ll INF=10000000000000000ll;
#define MAXN 2505
#define MAXM 30005
int v[MAXM],en,first[MAXN],next[MAXM];
ll cap[MAXM];
int d[MAXN],cur[MAXN];
queue<int>q;
int S,T;
void Init_Dinic(){memset(first,-1,sizeof(first)); en=0;}
void AddEdge(const int &U,const int &V,const ll &W)
{
    v[en]=V; cap[en]=W;
    next[en]=first[U]; first[U]=en++;
    v[en]=U; cap[en]=0;
    next[en]=first[V]; first[V]=en++;
}
bool bfs()
{
    memset(d,-1,sizeof(d)); q.push(S); d[S]=0;
    while(!q.empty())
      {
          int U=q.front(); q.pop();
          for(int i=first[U];i!=-1;i=next[i])
            if(d[v[i]]==-1 && cap[i])
              {
                d[v[i]]=d[U]+1;
                q.push(v[i]);
              }
      }
    return d[T]!=-1;
}
ll dfs(int U,ll a)
{
    if(U==T || !a) return a;
    ll Flow=0,f;
    for(int &i=cur[U];i!=-1;i=next[i])
      if(d[U]+1==d[v[i]] && (f=dfs(v[i],min(a,cap[i]))))
        {
          cap[i]-=f; cap[i^1]+=f;
          Flow+=f; a-=f; if(!a) break;
        }
    if(!Flow) d[U]=-1;
    return Flow;
}
ll max_flow()
{
    ll Flow=0,tmp=0;
    while(bfs())
      {
          memcpy(cur,first,sizeof(first));
          while(tmp=dfs(S,INF)) Flow+=tmp;
      }
    return Flow;
}
int n,m,pen,A,B,id[55][55];
char a[55][55];
int main(){
	scanf("%d%d%d%d",&n,&m,&A,&B);
	for(int i=1;i<=n;++i){
		for(int j=1;j<=m;++j){
			id[i][j]=++pen;
		}
	}
	for(int i=1;i<=n;++i){
		scanf("%s",a[i]+1);
	}
	S=pen+1;
	T=S+1;
	Init_Dinic();
	for(int i=1;i<=n;++i){
		for(int j=1;j<=m;++j){
			if(a[i][j]=='.'){
				AddEdge(S,id[i][j],(ll)B);
			}
			else{
				AddEdge(id[i][j],T,(ll)B);
			}
		}
	}
	for(int i=1;i<=n;++i){
		for(int j=1;j<=m;++j){
			for(int k=0;k<4;++k){
				int tx=i+dx[k],ty=j+dy[k];
				if(tx>=1 && tx<=n && ty>=1 && ty<=m){
					AddEdge(id[i][j],id[tx][ty],(ll)A);
				}
			}
		}
	}
	printf("%lld
",max_flow());
	return 0;
}