原题链接在这里：

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?

Related problem: 

题解：

类似. input变成 char array. 

先reverse 全部 char array. 从头往后走，遇到空格或者array尾部，就reverse中间的单词.

Time Compelxity: O(s.length()). Space: O(1).

AC Java:

1 public class Solution { 2     public void reverseWords(char[] s) { 3         if(s == null || s.length == 0){ 4             return; 5         } 6         reverse(s, 0, s.length-1); 7          8         int lo = 0; 9         for(int i = 1; i<=s.length; i++){10             if(i == s.length || s[i] == ' '){11                 reverse(s, lo, i-1);12                 lo = i+1;13             }14         }15     }16     17     private void reverse(char [] s, int i, int j){18         while(i < j){19             swap(s, i++, j--);20         }21     }22     23     private void swap(char [] s, int i , int j){24         char temp = s[i];25         s[i] = s[j];26         s[j] = temp;27     }28 }

跟上.