双向队列

声明一个双向队列，先入后出即可满足要求

class Solution {
public:
  string ReverseSentence(string str) {
      string tmp="";
      deque<string> de;
      for(auto strnum:str)
      {
          if(strnum==' ')
          {
              de.push_back(tmp);
              tmp="";
          }
          else
          {
              tmp=tmp+strnum;
          }
      }
      de.push_back(tmp);
      string result=de.back();
      de.pop_back();
      while(!de.empty())
      {
          result=result+" "+de.back();
          de.pop_back();
      }
      return result;

  }
};