参考 http://blog.youkuaiyun.com/u010902721/article/details/45771597
输出
set<int>::iterator it=a.begin();
for(int i=0;i<a.size()-1;i++)
{
cout<<*it<<" ";
it++;
}
for(it=a.begin();it!=a.end();it++)
{cout<<*it;}