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.

给你一个字符串，里面是一个个的单词，返回倒序的单词组合。

import java.util.Stack;

public class ReverseWordsinaString {
    /* * Given s = "the sky is blue", return "blue is sky the". */
    public static String reverseWords(String s) {
        Stack<String> st = new Stack<String>();
        // 直接使用split 效率不高
        int begin = 0;
        int end = s.length()-1;
        // 实现trim
        while (begin <= end && s.charAt(begin) == ' ') {
            begin++;
        }
        while (begin <= end && s.charAt(end) == ' ') {
            end--;
        }
        if (begin > end)
            return "";

        StringBuffer sb = new StringBuffer();
        while (begin <= end) {
            //每次都要首先去掉多余空格
            while (begin <= end && s.charAt(end) == ' ') {
                end--;
            }
            int tempEnd = end;
            while(begin <= end&&s.charAt(end) != ' ') {
                end--;
            }
            String tempString = s.substring(end+1, tempEnd + 1);
            sb.append(tempString + " ");
            end--;
        }

        sb.deleteCharAt(sb.length()-1);
        return sb.toString();
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String s = " a b ";
        System.out.print(reverseWords(s));
    }

}