Java - Permutation of ArrayList elements (Integer) - Can't get it to work properly

Question

I\'ve been looking around quite a bit to solve my issue. I got many problems solved but this one is still haunting me :S It\'s been a long time I haven\'t touch Java program

Answer 1

Try this, it seems to work, it uses recursion.

public class Permute {

    public static List<List<Integer>> permute(Integer...myInts){

        if(myInts.length==1){
            List<Integer> arrayList = new ArrayList<Integer>();
            arrayList.add(myInts[0]);
            List<List<Integer> > listOfList = new ArrayList<List<Integer>>();
            listOfList.add(arrayList);
            return listOfList;
        }

        Set<Integer> setOf = new HashSet<Integer>(Arrays.asList(myInts));   

        List<List<Integer>> listOfLists = new ArrayList<List<Integer>>();

        for(Integer i: myInts){
            ArrayList<Integer> arrayList = new ArrayList<Integer>();
            arrayList.add(i);

            Set<Integer> setOfCopied = new HashSet<Integer>();
            setOfCopied.addAll(setOf);
            setOfCopied.remove(i);

            Integer[] isttt = new Integer[setOfCopied.size()];
            setOfCopied.toArray(isttt);

            List<List<Integer>> permute = permute(isttt);
            Iterator<List<Integer>> iterator = permute.iterator();
            while (iterator.hasNext()) {
                List<java.lang.Integer> list = iterator.next();
                list.add(i);
                listOfLists.add(list);
            }
        }   

        return listOfLists;
    }

    public static void main(String[] args) {
        List<List<Integer>> permute = permute(1,2,3,4);
        System.out.println(permute);
    }

}

If you don't like the List> you can easily change from arrays to list using the methods from list and static methods from java.util.Collections and java.util.Arrays.

Answer 2

You can try Recursion to solve this issue:

public static void printPermutations(int[] n, int[] Nr, int idx) {
    if (idx == n.length) {  //stop condition for the recursion [base clause]
        System.out.println(Arrays.toString(n));
        return;
    }
    for (int i = 0; i <= Nr[idx]; i++) { 
        n[idx] = i;
        printPermutations(n, Nr, idx+1); //recursive invokation, for next elements
    }
}

More info can be had from this link: Combinatorics: generate all “states” - array combinations

You can replicate the same logic here as well.