1 /**
2 problem: http://poj.org/problem?id=2553
3 将所有出度为0环中的点排序输出即可。
4 **/
5
6 #include<stdio.h>
7 #include<stack>
8 #include<vector>
9 #include<algorithm>
10 using namespace std;
11
12 class Graphics{
13 const static int MAXN = 5005;
14 const static int MAXM = MAXN * MAXN;
15 private:
16 struct Edge{
17 int to, next;
18 }edge[MAXM];
19 struct Point{
20 int dfn, low, color;
21 Point(){dfn = low = color = 0;}
22 }point[MAXN], emptyPoint;
23 int first[MAXN], sign, colorNum, dfnNum, sumOfPoint;
24 bool vis[MAXN];
25 vector<int> ring[MAXN];
26 stack<int> stk;
27 void tarjan(int u){
28 point[u].dfn = ++ dfnNum;
29 point[u].low = dfnNum;
30 vis[u] = true;
31 stk.push(u);
32 for(int i = first[u]; i != -1; i = edge[i].next){
33 int to = edge[i].to;
34 if(!point[to].dfn){
35 tarjan(to);
36 point[u].low = min(point[to].low, point[u].low);
37 }else if(vis[to]){
38 point[u].low = min(point[to].dfn, point[u].low);
39 }
40 }
41 if(point[u].low == point[u].dfn){
42 vis[u] = false;
43 point[u].color = ++colorNum;
44 ring[colorNum].push_back(u);
45 while(stk.top() != u){
46 vis[stk.top()] = false;
47 point[stk.top()].color = colorNum;
48 ring[colorNum].push_back(stk.top());
49 stk.pop();
50 }
51 stk.pop();
52 }
53 }
54 public:
55 void clear(int n){
56 sign = colorNum = dfnNum = 0;
57 sumOfPoint = n;
58 for(int i = 1; i <= n; i ++){
59 first[i] = -1;
60 vis[i] = false;
61 ring[i].clear();
62 point[i] = emptyPoint;
63 }
64 while(!stk.empty()) stk.pop();
65 }
66 void addEdgeOneWay(int u, int v){
67 edge[sign].to = v;
68 edge[sign].next = first[u];
69 first[u] = sign ++;
70 }
71 void tarjanAllPoint(){
72 for(int i = 1; i <= sumOfPoint; i ++){
73 if(!point[i].dfn){
74 tarjan(i);
75 }
76 }
77 }
78 vector<int> getAns(){
79 vector<int> ans;
80 int *outdegree = new int[sumOfPoint+1];
81 for(int i = 1; i <= sumOfPoint; i ++){
82 outdegree[i] = 0;
83 }
84 tarjanAllPoint();
85 for(int i = 1; i <= sumOfPoint; i ++){
86 for(int j = first[i]; j != -1; j = edge[j].next){
87 int to = edge[j].to;
88 if(point[to].color != point[i].color){
89 outdegree[point[i].color] ++;
90 }
91 }
92 }
93 for(int i = 1; i <= colorNum; i ++){
94 if(!outdegree[i]){
95 for(int j = 0; j < ring[i].size(); j ++){
96 ans.push_back(ring[i][j]);
97 }
98 }
99 }
100 sort(ans.begin(), ans.end());
101 delete []outdegree;
102 return ans;
103 }
104 }graph;
105
106 int main(){
107 int n, m;
108 while(scanf("%d%d", &n, &m) != EOF && n){
109 graph.clear(n);
110 while(m --){
111 int a, b;
112 scanf("%d%d", &a, &b);
113 graph.addEdgeOneWay(a, b);
114 }
115 vector<int> ans = graph.getAns();
116 bool first = 1;
117 for(int i = 0; i < ans.size(); i ++){
118 if(first) first = 0;
119 else putchar(' ');
120 printf("%d", ans[i]);
121 }
122 putchar('\n');
123 }
124 return 0;
125 }
