Recursive method to permute characters in array Java

Question

I am trying to solve my homework. My task is to create an recursive method called void permute(char[] a, int lengthOfa) to permute the characters a,b,c,d like this:

• a b c d
• b a c d
• a c b d
• c a b d
• c b a d
• b c a d

This is the code I have come up with so far:

public class Main {
    public static void main(String[] args) {

        char[] charArray = {'a', 'b', 'c', 'd'};


        permute(charArray, 3);

    }

    public static void permute(char[] a, int lengthOfa) {

        System.out.println(a);

        if (lengthOfa <= 1) {
            return;
        }


        char temp = a[lengthOfa];

        a[1] = a[2];
        a[2] = temp;

        permute(a, lengthOfa - 1);


    }

}

How am I able to solve the task?

Answer 1

Thanks to the help of the comment of @Johnny Mopp I found a solution by myself:

public class Main2 {

public static void main(String[] args) {

    char[] charArray = {'a', 'b', 'c', 'd'};


    permute(charArray, charArray.length);

}

public static void permute(char[] a, int lengthOfa) {

    if (lengthOfa == 1) {
        printArray(a, 4);
    }

    for (int i = 0; i < lengthOfa; i++) {

        permute(a, lengthOfa - 1);

        if (lengthOfa % 2 == 1) {
            char temp = a[0];
            a[0] = a[lengthOfa - 1];
            a[lengthOfa - 1] = temp;

        } else {
            char temp = a[i];
            a[i] = a[lengthOfa - 1];
            a[lengthOfa - 1] = temp;


        }

    }

}

public static void printArray(char[] a, int n) {
    for (int i = 0; i < n; i++)
        System.out.print(a[i] + " ");
    System.out.println();
}

}