单词前后位置颠倒，大小写颠倒

public String trans(String s,int n) {
        //处理全部是空格的情况
        if(s.trim().length() == 0) return s;
        s = s.substring(0,n);       //校准s长度
        if(s.length() ==0) return s;//处理长度为0
        StringBuilder sb = new StringBuilder();
        s = changeStr(s); //大小写反转
        /*
         * 截取末单词置首
         */
        for (int i = s.length()-1; i >= 0; i--) {
            if(s.charAt(i)==' '){
                String str = s.substring(i+1);
                sb.append(str+" ");
                s=s.substring(0,i);
            }
        }
        sb.append(s);
        return sb.toString();
    }
    //大小写反转类
    public String changeStr(String s) {
        StringBuilder sb = new StringBuilder();
        for (char c : s.toCharArray()) {
            if (c <= 'z' && c >= 'a') {
                c = Character.toUpperCase(c);
            } else if (c <= 'Z' && c >= 'A') {
                c = Character.toLowerCase(c);
            }
            sb.append(c);
        }
        return sb.toString();
    }