tjut 1269

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/*2012-12-06 20:16:08   Accepted    1269    46MS    1484K   1339 B  C++*/  #include <cstdio>  #include <iostream>  #include <vector>  using namespace std;  #define maxn 10010    int n;  vector<int> g[maxn];    int Bcnt; /// 强连通分量的个数；  int Top; /// 栈顶；  int Index; /// 时间戳；  int low[maxn], dfn[maxn];  int belong[maxn], stack[maxn];  bool instack[maxn];    void Init_tarjan(){      Bcnt= Top= Index= 0;      for(int i=1; i<=n; ++i) low[i]= dfn[i]= 0;  }  void Tarjan(int u){      stack[Top++]= u;      instack[u]= 1;      low[u]= dfn[u]= ++Index;      for(int i=0; i<g[u].size(); ++i){          int v= g[u][i];          if( !dfn[v] ){              Tarjan(v);              low[u]= min( low[v], low[u] );          }          else if( instack[v] )              low[u]= min( low[u], dfn[v] );      }      if( low[u]==dfn[u] ){          ++Bcnt;          int v;          do{              v= stack[--Top];              instack[v]= 0;              belong[v]= Bcnt;          }while( u!=v );      }  }    int main()  {      int m;      while( cin>>n>>m, n+m ){          for(int i=1; i<=n; ++i) g[i].clear();          while(m--){              int x, y;              scanf("%d%d", &x, &y);              g[x].push_back(y);          }          Init_tarjan();          for(int i=1; i<=n; ++i){              if( !dfn[i] )                  Tarjan(1);          }            puts( Bcnt==1 ? "Yes" : "No" );      }  }
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