I have an arraylist of objects that I want to create all possible combinations (according to a simple set of rules). Each object that is stored in the list holds a squadNumber and a string. Here is an example of a typical list I am storing:
0: 1, A 1: 1, B 2: 2, A 3: 2, B 4: 3, C 5: 3, D 6: 4, C 7: 4, D
I want to get all the combinations where each squadNumber can only be present once, for example: (1,A),(2,A),(3,C),(4,C) then the next combination would be (1,A),(2,A),(3,C),(4,D). How would I go about this in java? Usually I would use a nested loop, but the fact that it's all being stored in one list complicates things for me.
Thanks, paintstripper
EDITED
Algorithm is following:
- Split all squads by numbers. So we have list with squads for 1, another list for squads 2, etc.
- Run dfs. At nth step we add squads with nth number.
Code
// Split squads by numbers, so we can iterate through each number independently. private Map<Integer, List<Squad>> splitSquadsByNumbers(List<Squad> squads) { Map<Integer, List<Squad>> res = new HashMap<Integer, List<Squad>>(); for (Squad squad : squads) { if (res.get(squad.getNumber()) == null) { res.put(squad.getNumber(), new ArrayList<Squad>()); } res.get(squad.getNumber()).add(squad); } return res; } List<Integer> squadNumbers; Map<Integer, List<Squad>> squadsByNumbers; Stack<Squad> stack; // Iterating through each squad with number squadNumbers[position] and try to add to stack, at the end pop it from stack. private void dfs(int position) { if (position == squadNumber.size()) { System.out.println(stack.toString()); } else { for (Squad squad : squadsByNumbers.get(squadNumber.get(position))) { stack.push(squad); dfs(position + 1); stack.pop(); } } } private void main(List<Squad> squads) { squadsByNumbers = splitSquadsByNumbers(squads); squadNumber = new ArrayList(squadsByNumber.keySet()); Collections.sort(squadNumbers); stack = new Stack<Squad>(); dfs(0); }
