2502: 清理雪道

题意：任意点出发任意次每条边至少经过一次最小花费。

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下界1，裸最小流....

#include <iostream>

#include <cstdio>

#include <cstring>

#include <algorithm>

using namespace std;

#define fir first

#define sec second

typedef long long ll;

const int N=1005, M=4e5+5, INF=1e9;

inline ll read(){

    char c=getchar();ll x=0,f=1;

    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}

    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}

    return x*f;

}

int n, m, x, s, t, tot, extra[N];

struct edge{int v, c, f, ne, lower;}e[M];

int cnt=1, h[N];

inline int ins(int u, int v, int c, int b=0) {

	e[++cnt]=(edge){v, c, 0, h[u], b}; h[u]=cnt;

	e[++cnt]=(edge){u, 0, 0, h[v], b}; h[v]=cnt;

	return cnt-1;

}

int q[N], head, tail, vis[N], d[N], cur[N];

bool bfs(int s, int t) {

	memset(vis, 0, sizeof(vis));

	head=tail=1;

	q[tail++]=s; d[s]=0; vis[s]=1;

	while(head!=tail) {

		int u=q[head++];

		for(int i=h[u];i;i=e[i].ne)

			if(!vis[e[i].v] && e[i].c>e[i].f) {

				vis[e[i].v]=1; d[e[i].v]=d[u]+1;

				q[tail++]=e[i].v;

				if(e[i].v == t) return true;

			}

	}

	return false;

}

int dfs(int u, int a, int t) {

	if(u==t || a==0) return a;

	int flow=0, f;

	for(int &i=cur[u];i;i=e[i].ne)

		if(d[e[i].v]==d[u]+1 && (f=dfs(e[i].v, min(a, e[i].c-e[i].f), t))>0) {

			flow+=f;

			e[i].f+=f;

			e[i^1].f-=f;

			a-=f;

			if(a==0) break;

		}

	if(a) d[u]=-1;

	return flow;

}

int dinic(int s, int t) {

	int flow=0;

	while(bfs(s, t)) {

		for(int i=0; i<=tot; i++) cur[i]=h[i];

		flow+=dfs(s, INF, t);

	}

	return flow;

}

int main() {

	freopen("in","r",stdin);

	n=read(); s=0; t=n+1;

	for(int i=1; i<=n; i++) {

		m=read();

		while(m--) x=read(), ins(i, x, INF, 1), extra[i]--, extra[x]++;

		ins(s, i, INF); ins(i, t, INF);

	}

	int ss=t+1, tt=t+2, sum=0; tot=tt;

	for(int i=s; i<=t; i++) {

		if(extra[i]>0) ins(ss, i, extra[i]), sum+=extra[i];

		if(extra[i]<0) ins(i, tt, -extra[i]);

	}

	ins(t, s, INF);

	int flow=dinic(ss, tt); //printf("flow %d\n",flow);

	int feas = e[cnt-1].f;

	e[cnt-1].c = e[cnt].c = 0;

	printf("%d",feas-dinic(t, s));

}