Leetcode - reverse string under specific requirement

输入一个string，把里面每个单词翻转，例如"this is a test", 输出"tset a si siht"
并且题目保证了string里只有小写字母和空格，并且有k个单词的话一定只有k - 1个空格

My code:

public String reverse(String s) {
        if (s == null || s.length() == 0)
                return s;
        StringBuilder sb = new StringBuilder();
        int begin = s.length() - 1;
        int end = s.length() - 1;
        while (end >= 0) {
            if (begin == end) {
                if (s.charAt(begin) == ' ') {
                    begin--;
                    end--;
                }
                else {
                    sb.append(s.charAt(end));
                    end--;
                }
            }
            else if (s.charAt(end) != ' ') {
                sb.append(s.charAt(end));
                end--;
            }
            else if (s.charAt(end) == ' ') {
                sb.append(" ");
                end--;
                begin = end;
            }
        }
        if (end < begin) {
            return sb.toString();
        }
        else {
            return sb.substring(0, sb.length() - 1);
        }
    }

没什么好说的。reverse string. 然后用 stringbuilder 倒序把他们收集起来。注意空格。

Anyway, Good luck, Richardo!