【CF1020E】Sergey's problem（构造）

题意：

思路：这是一道论文题 https://link.springer.com/content/pdf/10.1007/BFb0066192.pdf

From http://www.cnblogs.com/zhouzhendong/p/CF1019C.html

 #include<cstdio>
 #include<cstring>
 #include<string>
 #include<cmath>
 #include<iostream>
 #include<algorithm>
 #include<map>
 #include<set>
 #include<queue>
 #include<vector>
 #include<bitset>
 using namespace std;
 typedef long long ll;
 typedef unsigned int uint;
 typedef unsigned long long ull;
 typedef pair<int,int> PII;
 typedef vector<int> VI;
 #define fi first
 #define se second
 #define MP make_pair
 #define N      1100000
 #define M      51
 #define MOD 1000000007
 #define eps 1e-8
 #define pi     acos(-1)
 #define oo     3e14

 vector<int> c[N];
 int a[N],b[N],q[N];

 int main()
 {
     int n,m;
     scanf("%d%d",&n,&m);
     for(int i=;i<=n;i++) c[i].clear();
     for(int i=;i<=m;i++)
     {
         int x,y;
         scanf("%d%d",&x,&y);
         c[x].push_back(y);
     }
     for(int i=;i<=n;i++)
      if(!a[i])
      {
          for(int j=;j<=(int)c[i].size()-;j++) a[c[i][j]]=;
          a[i]=b[i]=;
      }
     for(int i=n;i>=;i--)
      if(b[i])
      {
          for(int j=;j<=(int)c[i].size()-;j++) b[c[i][j]]=;
      }
     int ans=;
     for(int i=;i<=n;i++)
      if(b[i]) q[++ans]=i;
     printf("%d\n",ans);
     for(int i=;i<=ans-;i++) printf("%d ",q[i]);
     printf("%d\n",q[ans]);
     return ;
 }