Leetcode 151 Reverse Words in a String

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.

Solution 1: 

 public class Solution {  
   public String reverseWords(String s) {  
     StringBuilder sb=new StringBuilder();  
     int n=s.length();  
     int j=n-1;  
     while (j>=0) {  
       while (j>=0 && s.charAt(j)==' ') j--;  
       int i=j;  
       while (i>=0 && s.charAt(i)!=' ') i--;  
       if (i<j) {  
         sb.append(s.substring(i+1,j+1));  
         sb.append(' ');  
       }  
       j=i-1;  
     }  
     int l=sb.length();  
     if (l==0) return "";  
     return sb.substring(0,l-1);  
   }  
 }