POJ 2553 The Bottom of a Graph

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <cmath>
 5 #include <algorithm>
 6 #include <climits>
 7 #include <vector>
 8 #include <queue>
 9 #include <cstdlib>
10 #include <string>
11 #include <set>
12 #include <stack>
13 #define LL long long
14 #define pii pair<int,int>
15 #define INF 0x3f3f3f3f
16 using namespace std;
17 const int maxn = 6000;
18 struct arc {
19     int to,next;
20     arc(int x = 0,int y = -1) {
21         to = x;
22         next = y;
23     }
24 };
25 arc e[maxn*100];
26 int head[maxn],dfn[maxn],low[maxn],belong[maxn],my[maxn];
27 int tot,n,m,top,scc,idx,out[maxn],ans[maxn];
28 bool instack[maxn];
29 void init() {
30     for(int i = 0; i < maxn; ++i) {
31         dfn[i] = low[i] = belong[i] = 0;
32         instack[i] = false;
33         head[i] = -1;
34         out[i] = 0;
35     }
36     scc = tot = idx = top = 0;
37 }
38 void add(int u,int v){
39     e[tot] = arc(v,head[u]);
40     head[u] = tot++;
41 }
42 void tarjan(int u) {
43     dfn[u] = low[u] = ++idx;
44     my[top++] = u;
45     instack[u] = true;
46     for(int i = head[u]; ~i; i = e[i].next) {
47         if(!dfn[e[i].to]) {
48             tarjan(e[i].to);
49             low[u] = min(low[u],low[e[i].to]);
50         } else if(instack[e[i].to]) low[u] = min(low[u],dfn[e[i].to]);
51     }
52     if(dfn[u] == low[u]) {
53         scc++;
54         int v;
55         do {
56             v = my[--top];
57             instack[v] = false;
58             belong[v] = scc;
59         } while(v != u);
60     }
61 }
62 int main() {
63     int u,v;
64     while(~scanf("%d",&n)&&n) {
65         scanf("%d",&m);
66         init();
67         for(int i = 0; i < m; ++i) {
68             scanf("%d%d",&u,&v);
69             add(u,v);
70         }
71         for(int i = 1; i <= n; ++i)
72             if(!dfn[i]) tarjan(i);
73         for(int i = 1; i <= n; ++i)
74             for(int j = head[i]; ~j; j = e[j].next)
75                 if(belong[i] != belong[e[j].to]) out[belong[i]]++;
76         int cnt = 0;
77         for(int i = 1; i <= n; ++i)
78             if(!out[belong[i]]) ans[cnt++] = i;
79         if(cnt){
80             for(int i = 0; i < cnt; ++i)
81                 printf("%d%c",ans[i],i + 1 == cnt?‘\n‘:‘ ‘);
82         }else puts("");
83     }
84     return 0;
85 }