[LeetCode] Problem 39 - Combination Sum

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Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note

  • All numbers (including target) will be positive integers.
  • The solution set must not contain duplicate combinations.

Example

No.1

Input: candidates = [2,3,6,7], target = 7,
A solution set is:

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[
  [7],
  [2,2,3]
]

No.2

Input: candidates = [2,3,5], target = 8,
A solution set is:

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[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

Code

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public List<List<Integer>> combinationSum(int[] candidates, int target) {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> combination = new ArrayList<>();
    Arrays.sort(candidates);

    combinationSumHelper(result, combination, candidates, target, 0);

    return result;
}

private void combinationSumHelper(List<List<Integer>> result, List<Integer> combination, int[] candidates, int target, int idx) {
    if (target < 0)
        return;
    
    if (target == 0) {
        result.add(new ArrayList<>(combination));
        return;
    }

    for (int i = idx; i < candidates.length; i++) {
        combination.add(candidates[i]);
        combinationSumHelper(result, combination, candidates, target - candidates[i], i);
        combination.remove(combination.size() - 1);
    }
}