leetcode------Reverse Words in a String

标题：	Reverse Words in a String
通过率：	14.8%
难度：	中等

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

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

Update (2015-02-12):
For C programmers: Try to solve it in-place in O(1) space.

click to show clarification.

Clarification:

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.

本本题思路比较清晰，就是遇到空格就是停止然后开始跳空格，最后进行迭代操作，具体代码如下：

 1 public class Solution {
 2     public String reverseWords(String s) {
 3            s = s.trim();
 4     return helper(s,0).toString();
 5         
 6     }
 7     private StringBuilder helper(String s, int index)
 8 {
 9     if(index>=s.length())
10         return new StringBuilder(); 
11     StringBuilder cur = new StringBuilder();
12     int lastIndex = index;
13     while(index < s.length() && s.charAt(index)!=' ')
14     {
15         cur.append(s.charAt(index++));
16     }
17     while(index < s.length() && s.charAt(index)==' ')
18         index++;
19     if(lastIndex == 0)
20         return helper(s,index).append(cur);
21     return helper(s,index).append(cur).append(' ');
22 }
23 }