仅提供个人的一种解题思路，未必是最优，仅供各位参考！

/**
 * 
 * <p>
 * ClassName SolutionReverseWordsInAString
 * </p>
 * <p>
 * Description Given an input string, reverse the string word by word.
 * 
 * For example, Given s = "the sky is blue", return "blue is sky the".
 * </p>
 * 
 * @author wangxu wangx89@126.com
 *         <p>
 *         Date 2014-9-11 下午01:55:53
 *         </p>
 * @version V1.0
 * 
 */
public class SolutionReverseWordsInAString {
	public static String reverseWords(String s) {

		if ("".equals(s.trim())) {
			return "";
		}
		String str[] = s.trim().split(" ");
		String result = "";
		int length = str.length - 1;
		for (int i = 0; i <= length / 2; i++) {
			if (!"".equals(str[i])) {

				String temp = str[i];
				str[i] = str[length - i];
				str[length - i] = temp;
			}
		}
		for (int i = 0; i <= length; i++) {
			if (!"".equals(str[i])) {

				if (i == length) {
					result += str[i];
				} else {
					result += str[i] + " ";
				}
			}

		}
		return result;
	}

	public static void main(String[] args) {
		System.out.println(reverseWords("   a   b "));
	}
}

Reverse Words In A String