hdu1269：强连通分量

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
using namespace std;
#define rep(i,n) for(int i=1;i<=n;i++)
#define clr(x,c) memset(x,c,sizeof(x))
#define adde(u,v) add(u,v),add(v,u)
const int nmax=10005;
int read(){
    int x=0,f=1;char c=getchar();
    while(!isdigit(c)){
        if(c==‘-‘) f=-1;
        c=getchar();
    }
    while(isdigit(c)){
        x=x*10+c-‘0‘;
        c=getchar();
    }
    return  x*f;
} 
struct edge{
    int from,to;
    edge *next;
    /*edge(int _from,int _to):
        from(_from),to(_to){};*/
};
edge e[100005],*pt,*head[nmax];
int sccno[nmax],pre[nmax],low[nmax],scc_cnt,dfn_clock,n,m;
void add(int u,int v){
    pt->to=v;pt->next=head[u];head[u]=pt++;
}
stack<int>S;
void dfs(int x){
    low[x]=pre[x]=++dfn_clock;
    S.push(x);
    for(edge *ee=head[x];ee;ee=ee->next){
        int to=ee->to;
        if(!pre[to]){
            dfs(to);
            low[x]=min(low[x],low[to]);
        }else if(!sccno[to]){
            low[x]=min(low[x],pre[to]);
        }
    }
    if(pre[x]==low[x]){
        scc_cnt++;
        while(1){
            int u=S.top();
            S.pop();
            sccno[u]=scc_cnt;
            if(u==x) break;
        }
    }
    return ;
}
void init(){
    clr(pre,0);
    dfn_clock=scc_cnt=0;
    clr(head,0);
    clr(sccno,0);
    pt=e;
}
int main(){
    while(scanf("%d%d",&n,&m)==2&&n+m){
        init();
        rep(i,m){
            int s=read(),t=read();
            add(s,t);
        }
        rep(i,n){
            if(!pre[i]) dfs(i);
         }
         if(scc_cnt==1) printf("Yes\n");
         else printf("No\n");
    }
    return 0;
}