用並查集寫的
題目有毒 n=1,m=0都能輸入,簡直了
#include<stdio.h>
#include<string.h>
int pre_forward[100010],pre_back[100010];
int n,m;
int find1(int a)
{
int r=a;
while(r!=pre_forward[r])
{
r=pre_forward[r];
}
return r;
}
int find2(int a)
{
int r=a;
while(r!=pre_back[r])
{
r=pre_back[r];
}
return r;
}
void create(int a,int b)
{
if(a!=n)
{
int fa=find1(a),fb=find1(b);
if(fa!=fb)
pre_forward[a]=b;
}
if(b!=n)
{
int fa=find2(a),fb=find2(b);
if(fa!=fb)
pre_back[b]=a;
}
}
int main()
{
while (scanf("%d%d",&n,&m),n||m)
{
int i,flag=1;
for(i=0;i<=n;i++)
{
pre_forward[i]=pre_back[i]=i;
}
for (i=1;i<=m;i++)
{
int a,b;
scanf("%d%d",&a,&b);
create(a,b);
}
for(i=1;i<=n;i++)
{
if(find1(i)!=n||find2(i)!=n)
{
flag=0;
break;
}
}
if(flag)
printf("Yes\n");
else
printf("No\n");
}
return 0;
}