Combination Sum

ID:39

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

It is guaranteed that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

Input: candidates = [2,3,6,7], target = 7
Output: [[2,2,3],[7]]
Explanation:
2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
7 is a candidate, and 7 = 7.
These are the only two combinations.

Idea

Recursion, in-place update

Construct recursive method "comb" with params: int[] candidates, int target, List<List>> result, List<>curr, int currIndex

If target==0 and curr not empty, a combination found, add to result

Iterate from currIndex, add currIndex element to curr, recursively call comb on curr with updated target value, if curr result in correct combination, execute base case above, else, return

After all element from a given index iterated, remove the last element from curr and go on iteration

Code

public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> output = new ArrayList<>();
        List<Integer> curr = new ArrayList<>();
        comb(candidates, target, output, curr, 0);
        return output;
}
    private void comb(int[] candidates, int target, List<List<Integer>> output, List<Integer> curr, int currInd){
        if(target==0 && !curr.isEmpty()){
            output.add(new ArrayList<>(curr));
        }
        else{
            for(int i = currInd; i<candidates.length; i++){
                if(target-candidates[i]>=0){
                    curr.add(candidates[i]);
                
                    comb(candidates, target-candidates[i], output, curr, i);
                    curr.remove(curr.size()-1);
                }
            }
        }
    }