Trapping Rain Water

You are given a list `height` where height[i] is the height of a wall at position i. After it rains, water collects in the dips between walls. At any position, the water level is limited by the tallest wall to its left and the tallest wall to its right. Compute the total number of units of water trapped. Input: `height` (a list of non-negative integers). Output: the total units of trapped water. Example: height = [0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1] -> 6.

def trap(height: list[int]) -> int:
    """Return how many units of rain water are trapped after raining."""
    pass