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;
}