leetcode/work/1/Hard/Array/42.trapping-rain-water.py

"""
42. Trapping Rain Water
Difficulty: Hard
https://leetcode.com/problems/trapping-rain-water/

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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 * 10^4

        • 0 <= height[i] <= 10^5
"""


class Solution:
    def trap(self, height: list[int]) -> int:
        left, right = 0, len(height) - 1
        leftMax, rightMax = 0, 0
        total = 0

        while left < right:
            leftMax = max(leftMax, height[left])
            rightMax = max(rightMax, height[right])

            if height[left] < height[right]:
                total += leftMax - height[left]
                left += 1
            else:
                total += rightMax - height[right]
                right -= 1

        return total