題意：

思路：這是一道論文題 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\n",ans);
57     for(int i=1;i<=ans-1;i++) printf("%d ",q[i]);
58     printf("%d\n",q[ans]);
59     return 0;
60 }