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。
下界1，裸最小流....
#include#include #include #include using namespace std;#define fir first#define sec secondtypedef 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));}