HDOJ 1269

//*************************************************************************//
//    Author: wangzhili
//     Mail:   wangstdio.h@gmail.com
//    Filename: qlt.c
//     Last modified: 2013-12-04 21:19
//*************************************************************************//

#include<stdio.h>
#include<string.h>
typedef struct
{
    int to;
    int next;
}EdgeNode;
EdgeNode edge[100005],redge[100005];
int head[10005],rhead[10005];
int vis[10005];
int t[10005];
int cnt;
void creatmap(int a,int b,int i)
{
    edge[i].to = b;
    edge[i].next = head[a];
    head[a] = i;
    redge[i].to = a;
    redge[i].next = rhead[b];
    rhead[b] = i;
}

void dfsone(int v)
{
    int j;
    vis[v] = 1;
    for(j = head[v];j != -1;j = edge[j].next)
    {
        if(!vis[edge[j].to])
            dfsone(edge[j].to);
    }
    t[++cnt] = v;
}

void dfstwo(int v)
{
    int j;
    vis[v] = 1;
    for(j = rhead[v];j != -1;j = redge[j].next)
    {
        if(!vis[redge[j].to])
            dfstwo(redge[j].to);
    }
}

int main()
{
    int n,m,i,j;
    int a,b;
    //freopen("/home/acmer/桌面/in.c","r",stdin);
    while(~scanf("%d%d",&n,&m) && (m+n))
    {
        cnt = 0;
        memset(head,-1,sizeof(head));
        memset(rhead,-1,sizeof(redge));
        memset(vis,0,sizeof(vis));
        memset(t,0,sizeof(t));
        for(i = 0;i < m;i ++)
        {
            scanf("%d%d",&a,&b);
            creatmap(a,b,i);
        }
        for(i = 1;i <= n;i ++)
        {
            if(!vis[i])
                dfsone(i);
        }
        memset(vis,0,sizeof(vis));
        cnt = 0;
        for(i = n;i >= 1;i --)
        {
            if(!vis[t[i]])
            {
                dfstwo(t[i]);
                cnt++;
            }
        }
        if(cnt == 1)
            printf("Yes\n");
        else
            printf("No\n");
    }
    return 0;
}