AC日记——[CQOI2009]DANCE跳舞 洛谷 P3153

[CQOI2009]DANCE跳舞

 

思路：

　　二分+最大流；

 

代码：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define maxn 505
#define maxm 30005
#define INF 0x3f3f3f3f
int head[maxn],E[maxm],V[maxm],F[maxm],cnt;
int n,k,deep[maxn],que[maxm<<1],s,t;
char map[maxn][maxn];
inline void edge_add(int u,int v,int f)
{
    E[++cnt]=head[u],V[cnt]=v,F[cnt]=f,head[u]=cnt;
    E[++cnt]=head[v],V[cnt]=u,F[cnt]=0,head[v]=cnt;
}
bool bfs()
{
    for(int i=s;i<=t;i++) deep[i]=-1;
    que[0]=s,deep[s]=0;int h=0,tail=1,now;
    while(h<tail)
    {
        now=que[h++];
        for(int i=head[now];i;i=E[i])
        {
            if(F[i]&&deep[V[i]]<0)
            {
                deep[V[i]]=deep[now]+1;
                if(V[i]==t) return true;
                que[tail++]=V[i];
            }
        }
    }
    return false;
}
int flowing(int now,int flow)
{
    if(now==t||flow<=0) return flow;
    int oldflow=0,pos;
    for(int i=head[now];i;i=E[i])
    {
        if(!F[i]||deep[V[i]]!=deep[now]+1) continue;
        pos=flowing(V[i],min(flow,F[i]));
        F[i]-=pos,F[i^1]+=pos,flow-=pos,oldflow+=pos;
        if(!flow) return oldflow;
    }
    if(!oldflow) deep[now]=-1;
    return oldflow;
}
bool check(int mid)
{
    cnt=1;
    for(int i=s;i<=t;i++) head[i]=0;
    for(int i=1;i<=n;i++) edge_add(s,i,mid),edge_add(i,i+n,k);
    for(int i=n*3+1;i<=n*4;i++) edge_add(i,t,mid),edge_add(i-n,i,k);
    for(int i=1;i<=n;i++)
        for(int v=1;v<=n;v++)
            if(map[i][v]=='Y') edge_add(i,v+n*3,1);
            else edge_add(i+n,n*2+v,1);
    int res=0;
    while(bfs()) res+=flowing(s,INF);
    return res==mid*n;
}
int main()
{
    scanf("%d%d",&n,&k),t=n*4+1;
    for(int i=1;i<=n;i++) scanf("%s",map[i]+1);
    int l=0,r=n,mid,ans=0;
    while(l<=r)
    {
        mid=l+r>>1;
        if(check(mid)) l=mid+1,ans=mid;
        else r=mid-1;
    }
    cout<<ans;
    return 0;
}