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
1 #include<cstdio> 2 #include<cstring> 3 #include<string> 4 #include<cmath> 5 #include<iostream> 6 #include<algorithm> 7 #include<map> 8 #include<set> 9 #include<queue> 10 #include<vector> 11 #include<bitset> 12 using namespace std; 13 typedef long long ll; 14 typedef unsigned int uint; 15 typedef unsigned long long ull; 16 typedef pair<int,int> PII; 17 typedef vector<int> VI; 18 #define fi first 19 #define se second 20 #define MP make_pair 21 #define N 1100000 22 #define M 51 23 #define MOD 1000000007 24 #define eps 1e-8 25 #define pi acos(-1) 26 #define oo 3e14 27 28 vector<int> c[N]; 29 int a[N],b[N],q[N]; 30 31 int main() 32 { 33 int n,m; 34 scanf("%d%d",&n,&m); 35 for(int i=1;i<=n;i++) c[i].clear(); 36 for(int i=1;i<=m;i++) 37 { 38 int x,y; 39 scanf("%d%d",&x,&y); 40 c[x].push_back(y); 41 } 42 for(int i=1;i<=n;i++) 43 if(!a[i]) 44 { 45 for(int j=0;j<=(int)c[i].size()-1;j++) a[c[i][j]]=1; 46 a[i]=b[i]=1; 47 } 48 for(int i=n;i>=1;i--) 49 if(b[i]) 50 { 51 for(int j=0;j<=(int)c[i].size()-1;j++) b[c[i][j]]=0; 52 } 53 int ans=0; 54 for(int i=1;i<=n;i++) 55 if(b[i]) q[++ans]=i; 56 printf("%d ",ans); 57 for(int i=1;i<=ans-1;i++) printf("%d ",q[i]); 58 printf("%d ",q[ans]); 59 return 0; 60 }