42.py

# leetcode problem no. 42(Hard ) 

class Solution:
    def trap(self, height: List[int]) -> int:
        
        if not height:
            return 0
        
        left, right = 0, len(height) - 1
        leftMax, rightMax = height[left], height[right]
        result = 0

        while left < right:
            if leftMax < rightMax:
                left += 1
                leftMax = max(leftMax, height[left]) 
                result += leftMax - height[left]
            else:
                right -= 1
                rightMax = max(rightMax, height[right])
                result += rightMax - height[right]
            
        return result