倒叙打印：比如abcdefg，打印结果gfedcba

    /*倒叙打印：比如abcdefg，打印结果gfedcba*/
    @Test
    public void recursion(){
        System.out.println(recursion("abcd"));
    }
    /*递归*/
    public static String recursion(String s) {
        int length = s.length();
        if(length <= 1){
            return s;
        }
        String left = s.substring(0, length / 2);
        String right = s.substring(length / 2, length);
        return recursion(right) + recursion(left);
    }