POJ 2762 Going from u to v or from v to u? （判断弱连通）

  1 #include<iostream>
  2 #include<algorithm>
  3 #include<cstring>
  4 #include<cstdio>
  5 #include<vector>
  6 #include<stack>
  7 #include<queue>
  8 #include<cmath>
  9 #include<map>
 10 using namespace std;
 11 typedef long long LL;
 12 
 13 const int maxn=6000+5;
 14 
 15 int n,m;
 16 
 17 int x[maxn],y[maxn];
 18 int in[maxn];
 19 int flag;
 20 
 21 vector<int> G[maxn];
 22 vector<int> new_G[maxn];
 23 
 24 stack<int> S;
 25 int pre[maxn],lowlink[maxn],sccno[maxn],dfs_clock,scc_cnt;
 26 
 27 void dfs(int u)
 28 {
 29     pre[u] = lowlink[u] = ++dfs_clock;
 30     S.push(u);
 31     for (int i = 0; i < G[u].size(); i++)
 32     {
 33         int v = G[u][i];
 34         if (!pre[v])
 35         {
 36             dfs(v);
 37             lowlink[u] = min(lowlink[u], lowlink[v]);
 38         }
 39         else if(!sccno[v])
 40         lowlink[u] = min(lowlink[u], pre[v]);
 41     }
 42     if (pre[u] == lowlink[u])
 43     {
 44         scc_cnt++;
 45         for(;;)
 46         {
 47             int x = S.top(); S.pop();
 48             sccno[x] = scc_cnt;
 49             if (x == u) break;
 50         }
 51     }
 52 }
 53 
 54 void find_scc(int n)
 55 {
 56     dfs_clock = scc_cnt = 0;
 57     memset(pre, 0, sizeof(pre));
 58     memset(sccno, 0, sizeof(sccno));
 59     for (int i = 1; i <= n; i++)
 60         if (!pre[i]) dfs(i);
 61 }
 62 
 63 
 64 void dfs2(int u)
 65 {
 66     if(flag==0)  return;
 67     if(new_G[u].size()>1)  {flag=0;return;}
 68     if(new_G[u].size()!=0) dfs2(new_G[u][0]);
 69 }
 70 
 71 
 72 void rebulid()
 73 {
 74     for(int i=1;i<=scc_cnt;i++)  {new_G[i].clear();in[i]=0;}
 75     for(int i=0;i<m;i++)
 76     {
 77         if(sccno[x[i]]!=sccno[y[i]])
 78         {
 79             new_G[sccno[x[i]]].push_back(sccno[y[i]]);
 80             in[sccno[y[i]]]=1;
 81         }
 82     }
 83     int num=0;
 84     int pos;
 85     for(int i=1;i<=scc_cnt;i++)
 86     {
 87         if(in[i]==0)  {num++;pos=i;}
 88         if(num>=2)  {flag=0;return;}
 89     }
 90     if(flag)  dfs2(pos);
 91 }
 92 
 93 int main()
 94 {
 95    //freopen("D:\\input.txt","r",stdin);
 96    int T;
 97    scanf("%d",&T);
 98    while(T--)
 99    {
100        flag=1;
101        scanf("%d%d",&n,&m);
102        for(int i=1;i<=n;i++)  G[i].clear();
103        for(int i=0;i<m;i++)
104        {
105            scanf("%d%d",&x[i],&y[i]);
106            G[x[i]].push_back(y[i]);
107        }
108        find_scc(n);
109        rebulid();
110        if(flag)  puts("Yes");
111        else puts("No");
112    }
113    return 0;
114 }