HDU 1269：迷宫城堡（强连通）

Posted 2020-08-12 Shadowdsp

http://acm.hdu.edu.cn/showproblem.php?pid=1269

题意：确定是否是一个强连通图。

思路：裸的tarjan算法。

 1 #include <cstdio>
 2 #include <algorithm>
 3 #include <iostream>
 4 #include <cstring>
 5 #include <string>
 6 #include <cmath>
 7 #include <queue>
 8 #include <vector>
 9 #include <stack>
10 using namespace std;
11 #define M 100010
12 #define N 10010
13 struct node
14 {
15     int v, next;
16 }edge[M];
17 int n, m, tot, cnt, head[N];
18 int dfn[N], low[N];
19 bool vis[N];
20 stack<int> sta;
21 
22 void init()
23 {
24     memset(dfn, 0, sizeof(dfn));
25     memset(low, 0, sizeof(low));
26     memset(head, -1, sizeof(head));
27     memset(vis, false, sizeof(vis));
28     tot = cnt = 0;
29     while(!sta.empty()) sta.pop();
30 }
31 
32 void add(int u, int v)
33 {
34     edge[tot].next = head[u]; edge[tot].v = v; head[u] = tot++;
35 }
36 
37 int tarjan(int u)
38 {
39     dfn[u] = low[u] = ++cnt;
40     sta.push(u);
41     vis[u] = true;
42     for(int i = head[u]; ~i; i = edge[i].next) {
43         int v = edge[i].v;
44         if(!dfn[v]) {
45             tarjan(v);
46             if(low[v] < low[u]) low[u] = low[v];
47         } else {
48             if(dfn[v] < low[u] && vis[v]) low[u] = dfn[v];
49         }
50     }
51     if(low[u] == dfn[u]) {
52         int top = 0;
53         int ans = 0;
54         while(top != u) { //注意一开始top等于u的情况这样ans会只算0,而不是1
55             ans++;
56             top = sta.top(); sta.pop();
57             vis[top] = false;
58         }
59         return ans;
60     }
61 }
62 
63 int main()
64 {
65     while(scanf("%d%d", &n, &m)) {
66         if(n + m == 0) break;
67         init();
68         for(int i = 0; i < m; i++) {
69             int u, v;
70             scanf("%d%d", &u, &v);
71             add(u, v);
72         }
73         int ans = tarjan(1);
74         if(n == ans) puts("Yes");
75         else puts("No");
76     }
77     return 0;
78 }