LeetCode-in-All

42. Trapping Rain Water

Hard

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Example 1:

Input: height = [0,1,0,2,1,0,1,3,2,1,2,1]

Output: 6

Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Example 2:

Input: height = [4,2,0,3,2,5]

Output: 9

Constraints:

• n == height.length
• 1 <= n <= 2 * 104
• 0 <= height[i] <= 105

Solution

defmodule Solution do
  @spec trap(h :: [integer]) :: integer
  def trap(h) do
    h = List.to_tuple(h)
    trap(h, 0, 0, 0, tuple_size(h) - 1, 0)
  end

  defp trap(_, _, _, idx_l, idx_r, acc) when idx_l > idx_r, do: acc
  defp trap(h, lmax, rmax, idx_l, idx_r, acc) when lmax <= rmax do
    current = elem(h, idx_l)
    new_lmax = max(lmax, current)
    trap(h, new_lmax, rmax, idx_l + 1, idx_r, acc + new_lmax - current)
  end

  defp trap(h, lmax, rmax, idx_l, idx_r, acc) when lmax > rmax do
    current = elem(h, idx_r)
    new_rmax = max(rmax, current)
    trap(h, lmax, new_rmax, idx_l, idx_r - 1, acc + new_rmax - current)
  end
end

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