Permutation algorithm for array of integers in Java

Question

I have a working example to generate all char permutations in a String as below:

static ArrayList permutations(String s) {
        if (s == nul         


        

Answer 1

import java.util.ArrayList;
import java.util.Arrays;

public class Answer {
    static  String arrayToString( E[] arr ) {
        final StringBuffer str = new StringBuffer();
        for ( E e : arr )
            str.append( e.toString() );
        return str.toString();
    }

    static  ArrayList permutations(E[] arr) {
        final ArrayList resultList = new ArrayList();
        final int l = arr.length;
        if ( l == 0 ) return resultList;
        if ( l == 1 )
        {
            resultList.add( arr );
            return resultList;
        }

        E[] subClone = Arrays.copyOf( arr, l - 1);
        System.arraycopy( arr, 1, subClone, 0, l - 1 );

        for ( int i = 0; i < l; ++i ){
            E e = arr[i];
            if ( i > 0 ) subClone[i-1] = arr[0];
            final ArrayList subPermutations = permutations( subClone );
            for ( E[] sc : subPermutations )
            {
                E[] clone = Arrays.copyOf( arr, l );
                clone[0] = e;
                System.arraycopy( sc, 0, clone, 1, l - 1 );
                resultList.add( clone );
            }
            if ( i > 0 ) subClone[i-1] = e;
        }
        return resultList;
    }

    static ArrayList permutations(String arr) {
        final Character[] c = new Character[ arr.length() ];
        for ( int i = 0; i < arr.length(); ++i )
            c[i] = arr.charAt( i );

        final ArrayList perms = permutations(c);
        final ArrayList resultList = new ArrayList( perms.size() );

        for ( Character[] p : perms )
        {
            resultList.add( arrayToString( p ) );
        }
        return resultList;
    }

    public static void main(String[] args) {
        ArrayList str_perms = permutations( "abc" );
        for ( String p : str_perms ) System.out.println( p );

        ArrayList int_perms = permutations( new Integer[]{ 1, 2, 3, 4 } );
        for ( Integer[] p : int_perms ) System.out.println( arrayToString( p ) );

    }
}