Daily Interview Problem: Trapping Rainwater

Hi, here's your problem today. This problem was recently asked by Uber:

You have a landscape, in which puddles can form. You are given an array of non-negative integers representing the elevation at each location. Return the amount of water that would accumulate if it rains.

For example: [0,1,0,2,1,0,1,3,2,1,2,1] should return 6 because 6 units of water can get trapped here.

       X               

   X███XX█X              

 X█XX█XXXXXX                   

# [0,1,0,2,1,0,1,3,2,1,2,1]

Here's your starting pont:

def capacity(arr):

  # Fill this in.



print capacity([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1])

# 6