You are given an input array whose each element represents the height of a line towers. The width of every tower is 1. It starts raining. How much water is collected between the towers?

Input: [1,5,3,7,2]
Output: 2 units

Explanation: 2 units of water collected between towers of height 5 and 7

Input: [5,3,7,2,6,4,5,9,1,2]
Output: 14 units

Explanation: 2 units of water collected between towers of height 5 and 7 +
             4 units of water collected between towers of height 7 and 6 +
             1 units of water collected between towers of height 6 and 5 +
             2 units of water collected between towers of height 6 and 9 +
             4 units of water collected between towers of height 7 and 9 +
             1 units of water collected between towers of height 9 and 2.