杭电1269----迷宫城堡

 1 //tarjan裸题
 2 #include <iostream>
 3 #include <vector>
 4 #include <stack>
 5 #include <cstring>
 6 using namespace std;
 7 const int maxn = 1e4+5;
 8 stack<int> s;
 9 vector<int> v[maxn];
10 int dfn[maxn],low[maxn],ins[maxn];
11 int ant,cnt;
12 int tarjan(int n)
13 {
14     dfn[n] = low[n] = ++ant;
15     s.push(n);
16     for(int i = 0; i < v[n].size(); ++i)
17     {
18         int x = v[n][i];
19         if(!dfn[x])
20             low[n] = min(low[n],tarjan(x));
21         else if(!ins[x])
22             low[n] = min(low[n],dfn[x]);
23     }
24     if(low[n] == dfn[n])
25     {
26         ++cnt;
27         int x;
28         do
29         {
30             x = s.top(); s.pop();
31             ins[x] = cnt;
32         }while(x != n);
33     }
34     return low[n];
35 }
36 int main()
37 {
38     int n,m,i;
39     while(cin >> n >> m && n+m)
40     {
41         ant = cnt = 0;
42         memset(dfn,0,sizeof dfn);
43         memset(low,0,sizeof low);
44         memset(ins,0,sizeof ins);
45         for(i = 0; i <= n; ++i)
46             v[i].clear();
47         while(m--)
48         {
49             int x,y;
50             cin >> x >> y;
51             v[x].push_back(y);
52         }
53         tarjan(1);
54         for(i = 1; i < n; ++i)
55             if(ins[i] != ins[i+1])
56                 break;
57         if(i == n)
58             cout << "Yes" << endl;
59         else
60             cout << "No" << endl;
61     }
62     return 0;
63 }