P4843 清理雪道

传送门

上下界网络流入门题...

每条边有一个下界流量 $1$，没有上界，求最小流

直接上下界最小流模板套进去就好了...

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<queue>
using namespace std;
typedef long long ll;
inline int read()
{
    int x=0,f=1; char ch=getchar();
    while(ch<'0'||ch>'9') { if(ch=='-') f=-1; ch=getchar(); }
    while(ch>='0'&&ch<='9') { x=(x<<1)+(x<<3)+(ch^48); ch=getchar(); }
    return x*f;
}
const int N=1e5+7,M=4e6+7,INF=1e9+7;
int fir[N],from[M],to[M],val[M],cntt=1;
inline void add(int a,int b,int c)
{
    from[++cntt]=fir[a]; fir[a]=cntt;
    to[cntt]=b; val[cntt]=c;
    from[++cntt]=fir[b]; fir[b]=cntt;
    to[cntt]=a; val[cntt]=0;
}
int n,SS,TT,sumflow[N],Ans;
int dep[N],Fir[N],S,T;
queue <int> q;
bool BFS()
{
    for(int i=0;i<=n+3;i++) Fir[i]=fir[i],dep[i]=0;
    q.push(S); dep[S]=1; int x;
    while(!q.empty())
    {
        x=q.front(); q.pop();
        for(int i=fir[x];i;i=from[i])
        {
            int &v=to[i]; if(dep[v]||!val[i]) continue;
            dep[v]=dep[x]+1; q.push(v);
        }
    }
    return dep[T]>0;
}
int DFS(int x,int mxf)
{
    if(x==T||!mxf) return mxf;
    int fl=0,res;
    for(int &i=Fir[x];i;i=from[i])
    {
        int &v=to[i]; if(dep[v]!=dep[x]+1||!val[i]) continue;
        if( res=DFS(v,min(mxf,val[i])) )
        {
            mxf-=res; fl+=res;
            val[i]-=res; val[i^1]+=res;
            if(!mxf) break;
        }
    }
    return fl;
}
inline int Dinic() { int res=0; while(BFS()) res+=DFS(S,INF); return res; }
inline void del(int x) { for(int i=fir[x];i;i=from[i]) val[i]=val[i^1]=0; }
int main()
{
    n=read();
    int a,b; S=0,T=n+1,SS=T+1,TT=SS+1;
    for(int i=1;i<=n;i++)
    {
        a=read();
        for(int j=1;j<=a;j++)
        {
            b=read(),add(i,b,INF);
            sumflow[i]--; sumflow[b]++;
        }
        add(S,i,INF); add(i,T,INF);
    }
    for(int i=1;i<=n;i++)
    {
        if(!sumflow[i]) continue;
        if(sumflow[i]<0) add(i,TT,-sumflow[i]);
        else add(SS,i,sumflow[i]);
    }
    add(T,S,INF);
    int tmpS=S,tmpT=T; S=SS,T=TT; Dinic();
    Ans=val[cntt]; val[cntt]=val[cntt^1]=0;
    del(SS),del(TT); S=tmpT,T=tmpS;
    printf("%d",Ans-Dinic());
    return 0;
}