BZOJ 2502: 清理雪道

有源汇有上下界最小流

#include<cstdio>#include<cstring>#include<algorithm>#include<cstdlib>#include<queue>#define INF 2e9#define N 110#define M 12500using namespace std; int read(){    int a=0,f=1;char c=getchar();    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}    while(c>='0'&&c<='9'){a=a*10+c-'0';c=getchar();}    return a*f;} struct edge{int to,next,v;}e[M*2];int n,m,cnt,sum,s,t,ss,tt;int a[N],d[N],head[N],cur[N]; void add(int x,int y,int v){    e[cnt].next=head[x],e[cnt].to=y,e[cnt].v=v;    head[x]=cnt++;    e[cnt].next=head[y],e[cnt].to=x,e[cnt].v=0;    head[y]=cnt++;} int bfs(){    queue<int>q;    memset(d,-1,sizeof(d));    q.push(ss),d[ss]=0;    while(!q.empty())    {        int x=q.front();q.pop();        if(x==tt) {memcpy(cur,head,sizeof(cur));return 1;}        for(int i=head[x];i!=-1;i=e[i].next)            if(d[e[i].to]==-1&&e[i].v)            {                d[e[i].to]=d[x]+1;                q.push(e[i].to);            }    }    return 0;} int dfs(int x,int a){    if(x==tt||!a) return a;    int flow=0,f;    for(int &i=cur[x];i!=-1;i=e[i].next)        if(d[e[i].to]==d[x]+1&&(f=dfs(e[i].to,min(a,e[i].v)))>0)        {            e[i].v-=f;            e[i^1].v+=f;            flow+=f;            a-=f;            if(!a) break;        }    if(!flow) d[x]=-1;    return flow; } int dinic(){    int res=0;    while(bfs())        res+=dfs(ss,INF);    return res;} int main(){    n=read();    s=0,t=n+1,ss=n+2,tt=n+3;    memset(head,-1,sizeof(head));    for(int i=1;i<=n;++i)        add(s,i,INF),add(i,t,INF);    for(int i=1,m;i<=n;++i)    {        m=read();        while(m--)        {            int j=read();            add(i,j,INF);//每个地方至少去一次即下界为1            --a[i],++a[j];        }    }    for(int i=1;i<=n;++i)        if(a[i]>0)            sum+=a[i],add(ss,i,a[i]);        else if(a[i]<0)            add(i,tt,-a[i]);    add(t,s,INF);    sum-=dinic();    sum=e[head[t]^1].v;    head[s]=e[head[s]].next,head[t]=e[head[t]].next;    ss=t,tt=s;    printf("%d\n",sum-dinic());    return 0;}

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