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.
<pre lang=py>
       X               
   X███XX█X              
 X█XX█XXXXXX                   
# [0,1,0,2,1,0,1,3,2,1,2,1]
</pre>

Here's your starting pont:
<pre lang=py>
def capacity(arr):
  # Fill this in.

print capacity([0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1])
# 6
</pre>