LeetCode in Elixir

39. Combination Sum

Medium

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.

Example 1:

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.

Example 2:

Input: candidates = [2,3,5], target = 8

Output: [[2,2,2,2],[2,3,3],[3,5]]

Example 3:

Input: candidates = [2], target = 1

Output: []

Constraints:

• 1 <= candidates.length <= 30
• 1 <= candidates[i] <= 200
• All elements of candidates are distinct.
• 1 <= target <= 500

Solution

defmodule Solution do
  @spec combination_sum(candidates :: [integer], target :: integer) :: [[integer]]
  def combination_sum(candidates, target) do
    combination_sum_rec(candidates, target, [], [])
  end

  defp combination_sum_rec(_, 0, sublist, ans) do
    [Enum.reverse(sublist) | ans]
  end

  defp combination_sum_rec([], _, _, ans) do
    ans
  end

  defp combination_sum_rec([h | t], target, sublist, ans) when target >= h do
    ans_with_h = combination_sum_rec([h | t], target - h, [h | sublist], ans)
    combination_sum_rec(t, target, sublist, ans_with_h)
  end

  defp combination_sum_rec([_ | t], target, sublist, ans) do
    combination_sum_rec(t, target, sublist, ans)
  end
end

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