Trapping Rain Water

Hard

Question

Given a list of non-negative integers representing the heights of walls, calculate the total amount of water that can be trapped after a rainfall.

Input: walls = [4, 2, 0, 3, 2, 3, 1]

Output: 5

Input: walls = [3, 1, 5, 2, 4, 2, 1]

Output: 4

What are some questions you'd ask an interviewer?

Given the walls map [0, 1, 0, 2, 0, 1], how much water can be trapped after rain?

1 unit of water

2 units of water

3 units of water

4 units of water