使用递归在Java中创建Mobius环

Question

So, the assignment requires the method public static String[] mobius(String s1, String s2). And a sample output is to be as follows, provided s1 = "Hello" and s2 = "Java":

HelloJava

elloJavaH

lloJavaHe

loJavaHel

oJavaHell

JavaHello

avaHelloJ

vaHelloJa

aHelloJav

This question ( Shifting characters in a string to the left ) got me started on the idea shown below, as the characters of each concatenated string are shifted left, much like an array shift. Here's what I have so far:

public static String[] mobius(String s1, String s2) {
    String combo = new String(s1 + s2);
    String[] mobius = new String[combo.length()];

    mobius[0] = s1 + s2;
    if (method4count < mobius.length) {

        mobius[method4count] = cyclicLeftShift(s1 + s2, method4count++);

        mobius(s1, s2);

    } else {
        return mobius;
    }
    return mobius;

}

public static String cyclicLeftShift(String s, int k) {
    k = k % s.length();
    return s.substring(k) + s.substring(0, k);
}

As of this question, the output is (When printing out the indexes of the String[]):

HelloJava

elloJavaH

and the remaining elements are null.

For reference, this is the "Test" Case that's been given to me:

String s1 = "Hello", s2 = "Java";
    String[] strs = mobius(s1, s2);
    for (String s : strs)
        System.out.println(s);

UPDATE: (Forgot to mention this initially, sorry about that) I am not allowed to use loops for this problem whatsoever. Thanks to those who provided the answers before!

Is there either an easier method (or just a method) to fill the remainder of the array (Not pertaining to the test case, but to the mobius method and to the cycleLeftShift method)? Or am I missing something that's probably obvious?

Thanks to anyone and everyone in advance! Also, sorry for the long "question" as it were

Answer 1

public class NewClass1 {

    private static int method4count = 1;
    static String[] mobius ;

    public static void main(String args[]){
        String s1 = "Hello", s2 = "Java";
        mobius = new String[(s1+s2).length()];

        String[] strs = mobius(s1, s2);        
        for (String s : strs)
            System.out.println(s);
    }
    public static String[] mobius(String s1, String s2) {      
        mobius[0] = s1 + s2;
        if (method4count < mobius.length) {
            mobius[method4count] = cyclicLeftShift(s1 + s2, method4count++);
            mobius(s1, s2);
        } else {
            return mobius;
        }
        return mobius;
    }

    public static String cyclicLeftShift(String s, int k) {
        k = k % s.length();
        return s.substring(k) + s.substring(0, k);
    }
}

Answer 2

private static void cycle(String s, int cases) {
    if (cases > 0) {
        String temp = cases == s.length() ? s : s.substring(1, s.length()) + s.charAt(0);
        System.out.println(temp);
        cycle(temp, cases - 1);
    }
}

This will perform what you request. It uses recursion and an integer denoting how many more cases it needs to perform. All it does is substring from index 1 to the end of the string and then append the current first letter to the back of the string.

Answer 3

Firstly, thanks to everyone who helped me solve the question!

Secondly; here is the answer I arrived at (from this help and the help of friends and more experienced coders:

static int method4count = 1;
static String combine = "";
static String[] array;

public static String[] mobius(String s1, String s2) {
    combine = s1 + s2;

    if (method4count == 1) {
        array = new String[combine.length()];
        array[0] = s1 + s2;
    }
    if (method4count < combine.length()) {
        array[method4count] = cyclicLeftShift(s1 + s2, method4count++);
        mobius(s1, s2);
    }

    return array;
}

public static String cyclicLeftShift(String s, int k) {
    k = k % s.length();
    return s.substring(k) + s.substring(0, k);
}

Test case is not included in the above work; what a TA and I arrived at was that with the static int method4count starting at 1, there is a conditional statement that, if returned true; initializes the String array to the length of the concatenated string "combine." Then the array is filled through a recursive call in the second if statement that checks to see if the counter is less than the length of "combine," and if true, assigns the next variant of the string in the mobius ring (through the method cyclicShiftLeft) before incrementing method4count up by one and recursively calling the mobius method. Once this array is filled and the second if statement proves to be false, then the array is returned.

Once again, thanks to everyone who helped with this problem!