rain array

You have an array of integers, for example: [ 9 8 7 6 2 ].

You graph the values of the array on the y-axis and the index on the x-axis as a bar graph.

Rain falls on the graph. The water collects in any valleys in the graph, and falls off any edges.

In the example array above no water collects because it all falls to the left and right. In [ 3 1 3 ] We get 2 units of water since it collects in the valley between the two 3 bars.

Devise an algorithm to determine how much water is collected by an array. 

Input: Array of integers

Output: Water collected when rain falls on graph

def rain(l=[]):
    result,  leftmaxSoFar, rightmaxSoFar  = 0,  0, 0
    leftMaxList = [0 for i in range(len(l))]
    rightMaxList = [0 for i in range(len(l))]
    capacity = [0 for i in range(len(l))]

    for i in range(len(l)):
        leftMaxList[i] =  leftmaxSoFar
        rightMaxList[len(l) -1 -i]  = rightmaxSoFar
        if l[i] > leftmaxSoFar :
           leftmaxSoFar = l[i]
        if l[len(l) -1 -i] > rightmaxSoFar:
           rightmaxSoFar = l[len(l) -1 -i]
            
    for i in range(len(l)):
        capacity[i] =  min(leftMaxList[i], rightMaxList[i]) -l[i]
        result = result if capacity[i] < 0 else result+capacity[i]
    return result

print rain([3, 1, 3])

print rain( [ 9, 8, 7, 6, 2 ])