UVa 12167 & HDU 2767 强连通分量 Proving Equivalences

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <algorithm>
 5 #include <vector>
 6 #include <stack>
 7 using namespace std;
 8 
 9 const int maxn = 20000 + 10;
10 
11 int n, m;
12 
13 vector<int> G[maxn];
14 
15 stack<int> S;
16 int pre[maxn], low[maxn], sccno[maxn];
17 int dfs_clock, scc_cnt;
18 
19 void dfs(int u, int fa)
20 {
21     low[u] = pre[u] = ++dfs_clock;
22     S.push(u);
23 
24     for(int i = 0; i < G[u].size(); i++)
25     {
26         int v = G[u][i];
27         if(!pre[v])
28         {
29             dfs(v, u);
30             low[u] = min(low[u], low[v]);
31         }
32         else if(!sccno[v]) low[u] = min(low[u], pre[v]);
33     }
34 
35     if(pre[u] == low[u])
36     {
37         scc_cnt++;
38         for(;;)
39         {
40             int x = S.top(); S.pop();
41             sccno[x] = scc_cnt;
42             if(x == u) break;
43         }
44     }
45 }
46 
47 void find_scc()
48 {
49     memset(pre, 0, sizeof(pre));
50     memset(sccno, 0, sizeof(sccno));
51     dfs_clock = scc_cnt = 0;
52     for(int i = 1; i <= n; i++) if(!pre[i]) dfs(i, 0);
53 }
54 
55 int in[maxn], out[maxn];
56 
57 int main()
58 {
59     int T; scanf("%d", &T);
60     while(T--)
61     {
62         scanf("%d%d", &n, &m);
63         for(int i = 1; i <= n; i++) G[i].clear();
64         while(m--)
65         {
66             int u, v; scanf("%d%d", &u, &v);
67             G[u].push_back(v);
68         }
69 
70         find_scc();
71 
72         if(scc_cnt == 1) { puts("0"); continue; }
73 
74         memset(in, 0, sizeof(in));
75         memset(out, 0, sizeof(out));
76         for(int i = 1; i <= n; i++)
77             for(int j = 0; j < G[i].size(); j++)
78             {
79                 int u = sccno[i], v = sccno[G[i][j]];
80                 if(u != v) { out[u]++; in[v]++; }
81             }
82 
83         int hehe = 0, haha = 0;
84         for(int i = 1; i <= scc_cnt; i++)
85         {
86             if(!in[i]) hehe++;
87             if(!out[i]) haha++;
88         }
89         printf("%d\n", max(hehe, haha));
90     }
91 
92     return 0;
93 }