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

代码

 public class Solution {

     public String reverseWords(String s) {

         // corner case

         if(s == null || s.length() == 0) return "";

         // s.trim() 去除leading or trailing spaces

         // s.split("\\s+")根据a single or multiple spaces 来split字符串

         String[] words = s.trim().split("\\s+");

         // use StringBuilder to save result

         StringBuilder sb = new StringBuilder();

         // to reverse, from right to left

         for(int i = words.length-1; i >= 0; i--){

             sb.append(words[i]+" ");

         }

         // 确保结果中去除了leading or trailing spaces

         return sb.toString().trim();

     }

 }