LeetCode-in-All

41. First Missing Positive

Hard

Given an unsorted integer array nums, return the smallest missing positive integer.

You must implement an algorithm that runs in O(n) time and uses constant extra space.

Example 1:

Input: nums = [1,2,0]

Output: 3

Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1]

Output: 2

Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12]

Output: 1

Explanation: The smallest positive integer 1 is missing.

Constraints:

Solution

defmodule Solution do
  @spec first_missing_positive(nums :: [integer]) :: integer
  def first_missing_positive(nums) do
    nums
    |> Enum.sort()
    |> find_missing_num()
  end

  defp find_missing_num(nums, acc \\ 1)

  defp find_missing_num([], acc) do
    acc
  end

  defp find_missing_num([h | t], acc) do
    acc = if h == acc, do: acc + 1, else: acc
    find_missing_num(t, acc)
  end
end