[LeetCode] Problem 77 - Combinations

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Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

Example

Input: n = 4, k = 2

Output:

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

Code

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public List<List<Integer>> combine(int n, int k) {
    List<List<Integer>> result = new ArrayList<>();

    combineHelper(result, new ArrayList<>(), n, k, 1);

    return result;
}

private void combineHelper(List<List<Integer>> result, List<Integer> combination, int n, int k, int idx) {
    if (combination.size() == k) {
        result.add(new ArrayList<>(combination));
        return;
    }

    for (int i = idx; i <= n; i++) {
        combination.add(i);
        combineHelper(result, combination, n, k, i + 1);
        combination.remove(combination.size() - 1);
    }
}