EZOJ #257

传送门

分析

先进行缩点

之后从终点倒着跑

对于一组边如果有一个点不能到达则这组边直接废掉

最后看只用没废掉的边能不能从起点走到终点

代码

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cctype>
#include<cmath>
#include<cstdlib>
#include<ctime>
#include<queue>
#include<vector>
#include<set>
#include<map>
#include<stack>
using namespace std;
int n,m,vis[],gone[],a[],t;
int sum,cnt,ist[],belong[],low[],dfn[];
stack<int>A;
vector<int>b[],v[],nv[],vv[];
inline void tarjan(int x){
    dfn[x]=low[x]=++cnt;
    ist[x]=;
    A.push(x);
    for(int i=;i<v[x].size();i++)
      if(!dfn[v[x][i]]){
          tarjan(v[x][i]);
          low[x]=min(low[x],low[v[x][i]]);
      }else if(ist[v[x][i]]){
          low[x]=min(low[x],dfn[v[x][i]]);
      }
    if(low[x]==dfn[x]){
      sum++;
      while(){
          int u=A.top();
          A.pop();
          ist[u]=;
          belong[u]=sum;
          if(u==x)break;
      }
    }
}
inline void dfs(int x){
    if(vis[x])return;
    vis[x]=;
    for(int i=;i<vv[x].size();i++)dfs(vv[x][i]);
}
inline void pr(){puts("YES");exit();}
inline void go(int x){
    if(gone[x])return;
    if(x==belong[t])pr();
    gone[x]=;
    for(int i=;i<nv[x].size();i++)go(nv[x][i]);
}
int main(){
    int i,j,k;
    scanf("%d%d",&n,&m);
    t=n+;
    for(i=;i<=m;i++){
      scanf("%d%d",&a[i],&k);
      for(j=;j<=k;j++){
          int x;
          scanf("%d",&x);
          b[i].push_back(x);
          v[a[i]].push_back(x);
      }
      if(!k)v[a[i]].push_back(t);
    }
    for(i=;i<=n;i++)
      if(!dfn[i])tarjan(i);
    for(i=;i<=n;i++)
      for(j=;j<v[i].size();j++)
        if(belong[i]!=belong[v[i][j]])
          vv[belong[v[i][j]]].push_back(belong[i]);
    dfs(belong[t]);
    for(i=;i<=m;i++){
      int ok=;
      for(j=;j<b[i].size();j++)
        if(!vis[belong[b[i][j]]]){
          ok=;
          break;
        }
      if(ok){
          for(j=;j<b[i].size();j++)
            nv[belong[a[i]]].push_back(belong[b[i][j]]);
          if(!b[i].size())nv[belong[a[i]]].push_back(belong[t]);
      }
    }
    go(belong[]);
    puts("NO");
    return ;
}