int main(){
      string s;
      cout<<"Please input a sentence:"<<endl;
      
      while(getline(cin,s)){
          vector<string> sentence=split(s);//split 是一个分割句子中单词的函数
      for(i=0;i!=sentence.size();++i)
          cout<<sentence[i]<<endl;
      }
      cout<<endl;
      cout<<"Please input a sentence:"<<endl;

      while(cin>>s)
           cout<<s<<endl;
     
      return 0;
  }
为什么在cmd中执行 while（getline(cin,s)) 这个循环总是不能退出，想问下如何 让getline(cin,s) 是假？