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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

# @param {Integer[]} height
# @return {Integer}
def trap(height)
  l = 0
  r = height.length - 1
  res = 0
  lower_wall = 0

  while l < r
    l_val = height[l]
    r_val = height[r]

    if l_val < r_val
      lower_wall = [l_val, lower_wall].max
      res += lower_wall - l_val
      l += 1
    else
      lower_wall = [r_val, lower_wall].max
      res += lower_wall - r_val
      r -= 1
    end
  end

  res
end

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