[luoguP2863] [USACO06JAN]牛的舞会The Cow Prom（Tarjan）

 1 #include <stack>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 
 6 const int MAXN = 50001;
 7 int n, m, cnt, idx, size, ans;
 8 int head[MAXN], to[MAXN << 1], next[MAXN << 1];
 9 int dfn[MAXN], low[MAXN], belong[MAXN], tot[MAXN];
10 bool ins[MAXN];
11 std::stack <int> s;
12 
13 inline int read()
14 {
15     int x = 0, f = 1;
16     char ch = getchar();
17     for(; !isdigit(ch); ch = getchar()) if(ch == ‘-‘) f = -1;
18     for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - ‘0‘;
19     return x * f;
20 }
21 
22 inline int min(int x, int y)
23 {
24     return x < y ? x : y;
25 }
26 
27 inline void add(int x, int y)
28 {
29     to[cnt] = y;
30     next[cnt] = head[x];
31     head[x] = cnt++;
32 }
33 
34 inline void dfs(int u)
35 {
36     int i, v;
37     s.push(u);
38     ins[u] = 1;
39     dfn[u] = low[u] = ++idx;
40     for(i = head[u]; i ^ -1; i = next[i])
41     {
42         v = to[i];
43         if(!dfn[v])
44         {
45             dfs(v);
46             low[u] = min(low[u], low[v]);
47         }
48         else if(ins[v]) low[u] = min(low[u], dfn[v]);
49     }
50     if(low[u] == dfn[u])
51     {
52         size++;
53         do
54         {
55             v = s.top();
56             s.pop();
57             ins[v] = 0;
58             belong[v] = size;
59         }
60         while(u ^ v);
61     }
62 }
63 
64 int main()
65 {
66     int i, x, y;
67     n = read();
68     m = read();
69     memset(head, -1, sizeof(head));
70     for(i = 1; i <= m; i++)
71     {
72         x = read();
73         y = read();
74         add(x, y);
75     }
76     for(i = 1; i <= n; i++)
77         if(!dfn[i])
78             dfs(i);
79     for(i = 1; i <= n; i++) tot[belong[i]]++;
80     for(i = 1; i <= size; i++)
81         if(tot[i] > 1)
82             ans++;
83     printf("%d\n", ans);
84     return 0;
85 }