BZOJ1718 [Usaco2006 Jan] Redundant Paths 分离的路径

 1 /**************************************************************
 2     Problem: 1718
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:44 ms
 7     Memory:3148 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11 #include <cstring>
12 #include <algorithm>
13  
14 using namespace std;
15 const int N = 5e4 + 5;
16 const int M = 1e5 + 5;
17  
18 struct edge {
19     int next, to;
20     edge(int _n = 0, int _t = 0) : next(_n), to(_t) {}
21 } e[M];
22  
23 int n, m, ans;
24 int cnt, top, num, tot = 1;
25 int dfn[N], low[N], vis[N], sz[N], s[N], w[N], first[N];
26 int mp[N];
27  
28 inline void Add_Edges(int x, int y) {
29     e[++tot] = edge(first[x], y), first[x] = tot;
30     e[++tot] = edge(first[y], x), first[y] = tot;
31 }
32  
33 void DFS(int p, int from) {
34     dfn[p] = low[p] = ++cnt;
35     s[++top] = p, vis[p] = 1;
36 #define y e[x].to
37     register int x;
38     for (x = first[p]; x; x = e[x].next)
39         if (x != (from ^ 1)) {
40             if (!vis[y]) DFS(y, x);
41             if (vis[y] < 2) low[p] = min(low[p], low[y]);
42         }
43 #undef y
44     if (dfn[p] == low[p]) {
45         register int y;
46         ++num;
47         while (s[top + 1] != p) {
48             y = s[top--];
49             vis[y] = 2, w[y] = num;
50             ++sz[num];
51         }
52     }
53 }
54  
55 inline void tarjan() {
56     int i;
57     cnt = top = num = 0;
58     memset(vis, sizeof(vis), 0);
59     for (i = 1; i <= n; ++i)
60         if (!vis[i]) DFS(i, 0);
61 }
62  
63 int main() {
64     int i, x, y;
65     scanf("%d%d", &n, &m);
66     for (i = 1; i <= m; ++i) {
67         scanf("%d%d", &x, &y);
68         Add_Edges(x, y);
69     }
70     tarjan();
71 #define y e[x].to
72     memset(mp, 0, sizeof(mp));
73     for (i = 1; i <= n; ++i)
74         for (x = first[i]; x; x = e[x].next)
75             if (w[i] != w[y]) ++mp[w[y]];
76     for (ans = i = 1; i <= num; ++i)
77         ans += (mp[i] == 1);
78     printf("%d\n", ans >> 1);
79 #undef y
80     return 0;