algorithm - Java: get all concatenations of List<List<String>>

Question

I have a List<List<String>> I need to get a List of all possible concatenation on the first dimension

[ [1,2 ], [1] , [3,4] ]

should give:

[ 113, 114, 213, 214 ]

I'm trying with 2 loops, like it should be possible to.

This is what I have tried:

private static List<String> constructIndexes(List<List<String>> indexList){
    List<String> index = new ArrayList<String>();

    String v="";

    for (int i=0; i< indexList.size(); i++){
        List<String> l =  indexList.get(i);
        for (int j=0; j<l.size(); j++){
            if (index.size()>0){
                for (int k=0; k<index.size(); k++){
                    index.set(k, index.get(k)+l.get(j));
                    // System.out.println(">");
                }
            } else {
                index.add(l.get(j));
            }

        }
    }

    return index;
}

some init code:

List<List<String>> indexList = new ArrayList<List<String>>();

    List<String> l = new ArrayList<String>();
    l.add("1");
    l.add("2");
    indexList.add(l);
    l = new ArrayList<String>();
    l.add("1");
    indexList.add(l);
    l = new ArrayList<String>();
    l.add("3");
    l.add("4");
    indexList.add(l);

    System.out.println( constructIndexes(indexList));

Answer 1

How about keeping some index counters which track the index of each element, then do traditional carrying to the left, the same you would do if adding, e.g. 9 + 1 = 10 (carry 1 to left, and set 0).

private static List<String> constructIndexes(List<List<String>> indexList) {
    List<String> index = new ArrayList<String>();
    int n = indexList.size();
    int[] counter = new int[n];
    int[] max = new int[n];
    int combinations = 1;
    for (int i = 0; i < n; i++) {
        max[i] = indexList.get(i).size();
        combinations *= max[i];
    }
    int nMinus1 = n - 1;
    for (int j = combinations; j > 0; j--) {
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < n; i++) {
            builder.append(indexList.get(i).get(counter[i]));
        }
        index.add(builder.toString());

        counter[nMinus1]++;
        for (int i = nMinus1; i >= 0; i--) {
            // overflow check
            if (counter[i] == max[i]) {
                if (i > 0) {
                    // carry to the left
                    counter[i] = 0;
                    counter[i - 1]++;
                }
            }
        }
    }
    return index;

Test

List<List<String>> test = Arrays.asList(Arrays.asList("1", "2"),
        Arrays.asList("1"), Arrays.asList("3", "4"));
System.out.println(constructIndexes(test));

Output

[113, 114, 213, 214]