Given an input string, reverse the string word by word.

For example,
Given s = "the sky is blue",
return "blue is sky the".

• What constitutes a word?
  A sequence of non-space characters constitutes a word.
• Could the input string contain leading or trailing spaces?
  Yes. However, your reversed string should not contain leading or trailing spaces.
• How about multiple spaces between two words?
  Reduce them to a single space in the reversed string.

題目解析：

題目中要刪除多餘的空格，有一種比較簡單的方法，先將s字串賦給一個臨時字串tmp，將頭尾空格略去，兩個字間的空格只複製一次。然後對tmp進行常規操作。這樣需要兩遍遍歷。

也可以臨時設一個結果字串，和一個臨時的字串。一點一點處理

class Solution {
public:
    void reverseWords(string &s) {
        string tmp;
        for(int i = s.size()-1;i >= 0;i--){
            while(i >= 0 && s[i] == ' ')
                i--;
            if(i<0)
                break;
            if(!tmp.empty())
                tmp.push_back(' ');
            string word;
            while(i>=0 && s[i]!=' ')
                word.push_back(s[i--]);
            reverse(word.begin(),word.end());
            tmp.append(word);
        }
        s = tmp;
    }
};