题目：

    Given an input string, reverse the string word by word. A word is defined as a sequence of non-space characters.

    The input string does not contain leading or trailing spaces and the words are always separated by a single space.

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

    Could you do it in-place without allocating extra space?

    思路就是两步走，第一步就是将整个字符串翻转。然后从头逐步扫描，将每个遇到单词再翻转过来。
    [注意事项]
    1）如果是Java，应该跟面试官指出String是immutable，所以需要用char array来做。
    2）follow-up问题：k-step reverse。也就是在第二部翻转的时候，把k个单词看作一个长单词，进行翻转。

    1. public  void  reverseWords(char []s){
    2.     // 将整个char数组翻转
    3.     reverseMethod(s, 0, s.length);
    4.     // 再将每个单词翻转 如果遇到"" 或 j = s.length,单
    5.     for(int i = 0, j = 0; j <=s.length; j++){
    6.         if(j == s.length || s[j] == ' '){
    7.             reverseMethod(s, i , j);
    8.             i = j + 1;
    9.         }
    10.     }
    11. }
    13. // 倒转方法
    14. private void  reverseMethod(char []s , int start , int end){
    15.      for(int i = 0; i < (start + end) / 2; i++){
    16.         char temp = s[end + i];
    17.         s[end  + i] = s[start + i];
    18.         s[start + i] = temp;
    19.     }
    21. }