Problem

Given a list of non-negative integers representing an elevation map where the width of each map is one, compute the amount of water it is able to collect after raining.

The above elavation map is represented by the array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water are being trapped. 6 units of water.

Sample input: [0,1,0,2,1,0,1,3,2,1,2,1] Sample output: 6

## solution
def trap(array):
    """Complexity: O(n) time | O(1) space"""
    trapped_water = 0
    left = 0
    right = len(array) - 1
    max_height = 0
    
    while left < right:
        if array[left] < array[right]:
            if array[left] > max_height:
                max_height = array[left]
            else:
                trapped_water += max_height - array[left]
            left += 1
        else:
            if array[right] > max_height:
                max_height = array[right]
            else:
                trapped_water += max_height - array[right]
            right -= 1
    return trapped_water

array = [0,1,0,2,1,0,1,3,2,1,2,1]
trap(array)

6

# unit tests
import unittest

class TrapTestCase(unittest.TestCase):
    def test_case_0(self):
        array = list(range(10))
        self.assertEqual(trap(array), 0)
        
    def test_case_1(self):
        array = [3, 1, 2, 0, 3, 0, 1, 2]
        self.assertEqual(trap(array), 9)
        
        

# if __name__ == "__main__":
#    unittest.main(argv=[""], exit=False)