42-trapping-rain-water.py

"""
42. Trapping Rain Water
Hard - 66.1%

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
"""

from typing import List

class Solution:
    def trap(self, height: List[int]) -> int:
        if not height:
            return 0
        
        left, right = 0, len(height) - 1
        left_max, right_max = 0, 0
        water = 0
        
        while left < right:
            if height[left] < height[right]:
                # Process left side
                if height[left] >= left_max:
                    left_max = height[left]
                else:
                    water += left_max - height[left]
                left += 1
            else:
                # Process right side
                if height[right] >= right_max:
                    right_max = height[right]
                else:
                    water += right_max - height[right]
                right -= 1
        
        return water