Question

I’m having a problem w/ my program. I have extracted a set of data and I would like to test if there is a combination for a particular number. For example, I have an array of int, 1 2 3 4 5, I would like to know if there is a combination for 7 maybe, and it must answer yes there is 3 + 4.

I figured out that I need to use the combination formula. So I thought that the outer loop may go like 5C1..5C2..5C3..etc, starting to “take 1” then “take 2” at a time to find out all the possible combinations. The problem is I’m stuck at how to implement this in actual codes.

I’m not really very good with Math, A defined loop structure would really help.

Thanks a lot in advance!

Answer 1

Here is a method that gets all possible sums from a List of Integers:

public static void getAllPermutations(final List<Integer> data,
    final Set<Integer> holder){

    if(data.isEmpty()){
        return;
    }
    final Integer first = data.get(0);
    if(data.size() > 1){
        getAllPermutations(data.subList(1, data.size()), holder);
        for(final Integer item : new ArrayList<Integer>(holder)){
            holder.add(first.intValue() + item.intValue());
        }
    }
    holder.add(first);
}

Usage:

List<Integer> data = Arrays.asList(1, 2, 3, 4, 5, 6);
Set<Integer> permutations = new TreeSet<Integer>();
getAllPermutations(data, permutations);
System.out.println(permutations);

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]

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While this solution won’t give you the operands that lead to the sum, it will include anything from 1 to 1 + 2 + 3 + 4 + 5 + 6