42) Trapping Rain Water.py

class Solution:
    def trap(self, height: List[int]) -> int:
        l =0
        r = len(height)-1
        lmax =0
        rmax =0
        ans = 0
        while l <r:
            if height[l] < height[r]:
                if height[l] >= lmax:
                    lmax = height[l]
                else:
                    ans += lmax - height[l]
                l +=1
            else:
                if height[r] >= rmax:
                    rmax = height[r]
                else:
                    ans += rmax-height[r]
                r -=1
        return ans