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".

 

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.

思路：采用String 的 split 方法进行分割 , 其分割的正则表达式为" ".

public class Solution {
    public String reverseWords(String s) {
        if (s == null) {
            return null;
        }
        s = s.trim();
        if (s.length() == 0) {
            return "";
        }
        String[] str = s.split(" ");
        StringBuilder sb = new StringBuilder();
        for (int i = str.length - 1; i >= 0; i--) {
            if (!str[i].equals(""))
            sb.append(str[i]).append(" ");
        }
        return sb.length() == 0 ? "" : sb.substring(0, sb.length() - 1);
    }
}