Trapping Rain Water – Isaac Flath

Problem Source: Leetcode

Problem Description

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.

tests

def test(fn):
    height = [0,1,0,2,1,0,1,3,2,1,2,1]
    actual = 6
    assert actual == fn(height) 

    height = [4,2,0,3,2,5]
    actual = 9
    assert actual == fn(height)

Solution

Time Complexity: O(n)
Space Complexity: O(1)

from typing import List
def trap(height: List[int]) -> int:
    l,r = 0, len(height)-1 # start on both ends
    vol = 0
    lm, rm = height[l], height[r]
    
    while l < r: # stop when pointers meet

        # The side with largest max determines the bin depth
        if lm < rm: 
            l += 1
            lm = max(height[l], lm)
            vol += lm - height[l] 
        else:
            r -= 1 
            rm = max(height[r], rm) 
            vol += rm - height[r]
    return vol

test(trap)