BZOJ1654 [Usaco2006 Jan]The Cow Prom 奶牛舞会

 1 /**************************************************************
 2     Problem: 1654
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:32 ms
 7     Memory:1476 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11 #include <cstring>
12 #include <algorithm>
13  
14 using namespace std;
15 const int N = 10005;
16 const int M = 50005;
17 struct edges{
18     int next, to;
19 }e[M];
20 int n, m;
21 int cnt, top, num, tot, ans, sz;
22 int dfn[N], low[N], vis[N], s[N], first[N];
23  
24 inline int read(){
25     int x = 0, sgn = 1;
26     char ch = getchar();
27     while (ch < ‘0‘ || ch > ‘9‘){
28         if (ch == ‘-‘) sgn = -1;
29         ch = getchar();
30     }
31     while (ch >= ‘0‘ && ch <= ‘9‘){
32         x = x * 10 + ch - ‘0‘;
33         ch = getchar();
34     }
35     return sgn * x;
36 }
37  
38 inline void add_edge(int x, int y){
39     e[++tot].next = first[x];
40     first[x] = tot;
41     e[tot].to = y;
42 }
43  
44 void DFS(int p){
45     dfn[p] = low[p] = ++cnt;
46     s[++top] = p, vis[p] = 1;
47     int x, y;
48     for (x = first[p]; x; x = e[x].next){
49         y = e[x].to;
50         if (!vis[y]) DFS(y);
51         if (vis[y] < 2)
52             low[p] = min(low[p], low[y]);
53     }
54     if (dfn[p] == low[p]){
55         ++num, sz = 0;
56         while (s[top + 1] != p){
57             y = s[top--];
58             vis[y] = 2;
59             ++sz;
60         }
61         if (sz != 1) ++ ans;
62     }
63 }
64  
65 inline void tarjan(){
66     cnt = top = num = 0;
67     memset(vis, sizeof(vis), 0);
68     for (int i = 1; i <= n; ++i)
69         if (!vis[i]) DFS(i);
70 }
71  
72 int main(){
73     n = read(), m = read();
74     int x, y;
75     for (int i = 1; i <= m; ++i){
76         x = read(), y = read();
77         add_edge(x, y);
78     }
79     tarjan();
80     printf("%d\n", ans);
81     return 0;
82 }