Leetcode (Python): Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example, 
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.

Solution:

class Solution:
    # @param A, a list of integers
    # @return an integer
    def trap(self, A):
        max = 0
        maxIndex = 0
        for i in range(0,len(A)):
            if A[i] > max:
                max = A[i]
                maxIndex = i
                
        previousMax = 0
        count = 0
        for i in range(0, maxIndex):
            if A[i] > previousMax:
                previousMax = A[i]
            count += previousMax - A[i]
        
        previousMax = 0
        for i in range(len(A)-1, maxIndex, -1):
            if A[i] > previousMax:
                previousMax = A[i]
            count += previousMax - A[i]
        return count