hdu 1269 迷宫城堡 (强连通入门题）

/******************************************************
       迷宫城堡(hdu 1269)
       强连通入门题

******************************************************/


#include<cstdio>
#include<cstring>
#include<algorithm>
#include<stack>
#include<vector>
using namespace std;

const int mx=10005;
stack<int>s;
vector<int>g[mx];
int dfn[mx],low[mx];
int sccno[mx],vs[mx];///sccno记录每个节点是第几个强连通分量
int cut_dfs,cut_scc; ///cut_dfn记录dfs每个节点遍历循序，
                     ///cut_scc记录强连通分量的个数

void dfs(int u)
{
    s.push(u);
    vs[u]=1;
    dfn[u]=low[u]=++cut_dfs;
    for (int i=0;i<g[u].size();i++)
    {
        int v=g[u][i];
        if (!vs[v])
        {
            dfs(v);
            low[u]=min(low[u],low[v]);
        }
        else low[u]=min(low[u],dfn[v]);
    }
    if (low[u]==dfn[u])
    {
        cut_scc++;
        while (1)
        {
            int x=s.top();
            s.pop();
            sccno[x]=cut_scc;
            if (x==u) break;
        }
    }
}

int main()
{
    int n,m,a,b;
    while (~scanf("%d%d",&n,&m)&&n+m)
    {
        for (int i=1;i<=n;i++) g[i].clear();
        while (m--)
        {
            scanf("%d%d",&a,&b);
            g[a].push_back(b);
        }
        while (!s.empty()) s.pop();
        cut_dfs=cut_scc=0;
        memset(dfn,0,sizeof(dfn));
        memset(low,0,sizeof(low));
        memset(vs,0,sizeof(vs));
        for (int i=1;i<=n;i++)
        {
            if (!dfn[i]) dfs(i);
        }
        if (cut_scc==1) printf("Yes
");
        else printf("No
");
    }
}