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

Example:

Input: "the sky is blue",
Output: "blue is sky the".
Note:

A word is defined as a sequence of non-space characters.
Input string may contain leading or trailing spaces. However, your reversed string should not contain leading or trailing spaces.
You need to reduce multiple spaces between two words to a single space in the reversed string.
Follow up: For C programmers, try to solve it in-place in O(1) space.

思路

1.  split by a single or multiple spaces, making sure remove leading or trailing spaces

2. to reverse,  traverse the input from right to left,  append each item into StringBuilder

程式碼

 1 public class Solution {
 2     public String reverseWords(String s) {
 
 3         // corner case 
 4         if(s == null || s.length() == 0) return "";
 5         // s.trim() 去除leading or trailing spaces
 6         // s.split("\\s+")根據a single or multiple spaces 來split字串
 7         String[] words = s.trim().split("\\s+");
 8         // use StringBuilder to save result 
 9         StringBuilder sb = new
 
 StringBuilder();
10         // to reverse, from right to left 
11         for(int i = words.length-1; i >= 0; i--){
12             sb.append(words[i]+" "); 
13         }
14         // 確保結果中去除了leading or trailing spaces
15         return sb.toString().trim();
16     }
17 }