Trapping Rain Water

Problem

https://leetcode.com/problems/trapping-rain-water/

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 * 10:sup:`4`

• 0 <= height[i] <= 10:sup:`5`

Pattern

Array, Two Pointers, Dynamic Programming, Stack, Monotonic Stack

Approaches

Two Pointers

Code

def trap(height: list[int]) -> int:
    left = 0
    right = len(height) - 1

    left_max = height[left]
    right_max = height[right]

    trapped = 0
    while left < right:
        if height[left] <= height[right]:
            left += 1
            left_max = max(left_max, height[left])
            trapped += left_max - height[left]

        else:
            right -= 1
            right_max = max(right_max, height[right])
            trapped += right_max - height[right]

    return trapped

Test

>>> from trapping_rain_water__two_pointers import trap
>>> trap([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1])
6
>>> trap([4, 2, 0, 3, 2, 5])
9

trapping_rain_water__two_pointers.trap(height: list[int]) → int