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.
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 * 104
0 <= height[i] <= 105
Solution
Python3
class Solution:
def trap(self, height: List[int]) -> int:
N = len(height)
res = 0
left, right = 0, N - 1
leftMost, rightMost = height[0], height[-1]
while left < right:
leftMost = max(leftMost, height[left])
rightMost = max(rightMost, height[right])
if leftMost <= rightMost:
res += leftMost - height[left]
left += 1
else:
res += rightMost - height[right]
right -= 1
return res