LeetCode-in-All

239. Sliding Window Maximum

Hard

You are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position.

Return the max sliding window.

Example 1:

Input: nums = [1,3,-1,-3,5,3,6,7], k = 3

Output: [3,3,5,5,6,7]

Explanation:

Window position Max


[1 3 -1] -3 5 3 6 7 3

1 [3 -1 -3] 5 3 6 7 3

1 3 [-1 -3 5] 3 6 7 5

1 3 -1 [-3 5 3] 6 7 5

1 3 -1 -3 [5 3 6] 7 6

1 3 -1 -3 5 [3 6 7] 7

Example 2:

Input: nums = [1], k = 1

Output: [1]

Constraints:

Solution

defmodule Solution do
  @spec max_sliding_window(nums :: [integer], k :: integer) :: [integer]
  def max_sliding_window(nums, k) do
    Enum.with_index(nums)
    |> Enum.map_reduce(:queue.new(), fn {x, i}, q ->
      enqueue(q, x, i)
      |> get_max(i - k)
    end)
    |> elem(0)
    |> Enum.drop(k - 1)
  end

  defp enqueue(q, x, i) do
    case :queue.peek_r(q) do
      {:value, {y, _}} when y <= x ->
        :queue.drop_r(q)
        |> enqueue(x, i)
      _ -> :queue.in({x, i}, q)
    end
  end

  defp get_max(q, drop_until) do
    case :queue.peek(q) do
      {:value, {_, j}} when j <= drop_until ->
        :queue.drop(q)
        |> get_max(drop_until)
      {:value, {y, _}} -> {y, q}
    end
  end
end