#最小路徑覆蓋問題(網絡流最大流) ##題目 洛谷題目傳送門 ##題解 網絡流題目詳講 ##codehtml
#include<iostream> #include<cstdlib> #include<cstdio> #include<cmath> #include<cstring> #include<iomanip> #include<algorithm> #include<ctime> #include<queue> #include<stack> #include<vector> #define rg register #define il inline #define lst long long #define ldb long double #define N 550 #define M 100050 using namespace std; const int Inf=1e9; il int read() { rg int s=0,m=0;rg char ch=getchar(); while(ch<'0'||ch>'9'){if(ch=='-')m=1;ch=getchar();} while(ch>='0'&&ch<='9')s=(s<<3)+(s<<1)+(ch^48),ch=getchar(); return m?-s:s; } int n,m,S,T,ans; int to[N],tag[N],dep[N]; int hd[N],cur[N],cnt=1; struct EDGE{int to,nxt,c;}ljl[M<<1]; il void add(rg int p,rg int q,rg int o) { ljl[++cnt]=(EDGE){q,hd[p],o},hd[p]=cnt; ljl[++cnt]=(EDGE){p,hd[q],0},hd[q]=cnt; } queue<int> Q; il bool BFS() { for(rg int i=S;i<=T;++i)dep[i]=0; while(!Q.empty())Q.pop(); Q.push(S),dep[S]=1; while(!Q.empty()) { rg int now=Q.front();Q.pop(); for(rg int i=hd[now];i;i=ljl[i].nxt) { rg int qw=ljl[i].to; if(!dep[qw]&&ljl[i].c>0) { dep[qw]=dep[now]+1; Q.push(qw); } } } return dep[T]; } int dfs(rg int now,rg int aim,rg int flow) { if(now==aim)return flow; for(rg int &i=cur[now];i;i=ljl[i].nxt) { rg int qw=ljl[i].to; if(ljl[i].c>0&&dep[qw]==dep[now]+1) { rg int kk=dfs(qw,aim,min(flow,ljl[i].c)); if(kk>0) { to[now]=qw; if(now!=S)tag[qw-n]=1; ljl[i].c-=kk,ljl[i^1].c+=kk; return kk; } } } return 0; } il int Dinic() { rg int ans=0; while(BFS()) { for(rg int i=S;i<=T;++i)cur[i]=hd[i]; while(int kk=dfs(S,T,Inf))ans+=kk; } for(rg int i=1;i<=n;++i) if(!tag[i]) { rg int now=i; printf("%d ",now); while(to[now]&&to[now]!=T) { printf("%d ",to[now]-n); now=to[now]-n; } puts(""); } return ans; } int main() { n=read(),m=read(); S=0,T=2*n+1; for(rg int i=1;i<=n;++i) add(S,i,1),add(i+n,T,1); for(rg int i=1;i<=m;++i) { rg int p=read(),q=read(); add(p,q+n,1); } printf("%d\n",n-Dinic()); return 0; }