轻车熟路

思路之一如下：
1、按空格划分成字符串数组
2、数组元素顺序入栈
3、数组元素逆序出栈
4、大小写互转

import java.util.*;

public class Transform {
    public String trans(String s, int n) {
        //空格划分成数组
        String[] arr =  s.split(" ",-1);
        //数组元素入栈
        Stack<String> stack = new Stack<String>();
        for(String str:arr){
            stack.push(str);
        }
        //逆序出栈
        StringBuilder sb = new StringBuilder();
        while(!stack.isEmpty()){
             sb.append(stack.pop()+" ");
        }
        //去除最后一个空格
        sb.deleteCharAt(sb.length()-1);
        //大小写互换
        return reverse(sb.toString());
    }

    public String reverse(String s){
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
                    char temp = s.charAt(i);
                    if (temp >= 'a' && temp <= 'z') {
                        temp = (char)(temp-32);
                    }else if(temp >= 'A' && temp <= 'Z') {
                        temp = (char)(temp+32);
                    }
            sb.append(temp);
        }
        return sb.toString();
    }
}