hdu 1269 迷宫城堡

#include <cstring>
#include <cstdio>
#include <vector>
#define N 100500

using std::vector;
int min(int a,int b) {return a>b?b:a;}
vector<int>edge[N];
bool instack[N];
int Map[1005][1005],n,m,dfn[N],stack[N],low[N],tim,top,sumcol;
inline void init()
{
    sumcol=top=tim=0;
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
}
void tarjan(int x)
{
    stack[++top]=x;
    instack[x]=true;
    dfn[x]=low[x]=++tim;
    for(int i=0;i<edge[x].size();++i)
    {
        int v=edge[x][i];
        if(instack[v]) low[x]=min(low[x],dfn[v]);
        else if(!dfn[v])
        {
            tarjan(v);
            low[x]=min(low[x],low[v]);
        }
    }
    if(low[x]==dfn[x])
    {
        sumcol++;
        int k;
        do
        {
            k=stack[top--];
            instack[k]=false;
        }while(k!=x);
    }
}
int main()
{
    for(;scanf("%d%d",&n,&m)&&n;)
    {
        for(int x,y;m--;)
        {
            scanf("%d%d",&x,&y);
            edge[x].push_back(y);
        }
        for(int i=1;i<=n;++i)
            if(!dfn[i]) tarjan(i);
        if(sumcol==1) printf("Yes\n");
        else printf("No\n");
        for(int i=1;i<=n;++i) edge[i].clear();
        init();
    }
    return 0;
}