HDU 3836 Equivalent Sets

 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 = 20100;
18 int dfn[maxn],low[maxn],belong[maxn];
19 bool instack[maxn];
20 vector<int>g[maxn];
21 stack<int>stk;
22 int n,m,cnt,scc,in[maxn],out[maxn];
23 void tarjan(int u){
24     dfn[u] = low[u] = ++cnt;
25     stk.push(u);
26     instack[u] = true;
27     for(int i = 0; i < g[u].size(); i++){
28         if(!dfn[g[u][i]]){
29             tarjan(g[u][i]);
30             low[u] = min(low[u],low[g[u][i]]);
31         }else if(instack[g[u][i]]) low[u] = min(low[u],dfn[g[u][i]]);
32     }
33     if(dfn[u] == low[u]){
34         scc++;
35         int v;
36         do{
37             v = stk.top();
38             instack[v] = false;
39             belong[v] = scc;
40             stk.pop();
41         }while(v != u);
42     }
43 }
44 int main() {
45     int i,j,u,v,a,b;
46     while(~scanf("%d %d",&n,&m)){
47         for(i = 0; i <= n; i++){
48             dfn[i] = low[i] = belong[i] = 0;
49             instack[i] = false;
50             g[i].clear();
51             in[i] = out[i] = 0;
52         }
53         cnt = scc = 0;
54         while(!stk.empty()) stk.pop();
55         for(i = 1; i <= m; i++){
56             scanf("%d %d",&u,&v);
57             g[u].push_back(v);
58         }
59         for(i = 1; i <= n; i++)
60             if(!dfn[i]) tarjan(i);
61         for(i = 1; i <= n; i++){
62             for(j = 0; j < g[i].size(); j++){
63                 if(belong[i] != belong[g[i][j]]){
64                     in[belong[g[i][j]]]++;
65                     out[belong[i]]++;
66                 }
67             }
68         }
69         for(a = b = 0,i = 1; i <= scc; i++){
70             if(!in[i]) a++;
71             if(!out[i]) b++;
72         }
73         printf("%d\n",scc == 1?0:max(a,b));
74     }
75     return 0;
76 }