codility 5-2. PassingCars

A non-empty zero-indexed array A consisting of N integers is given. The consecutive elements of array A represent consecutive cars on a road. Array A contains only 0s and/or 1s: 0 represents a car traveling east, 1 represents a car traveling west. The goal is to count passing cars. We say that a pair of cars (P, Q), where 0 ≤ P < Q < N, is passing when P is traveling to the east and Q is traveling to the west. For example, consider array A such that: A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1 We have five pairs of passing cars: (0, 1), (0, 3), (0, 4), (2, 3), (2, 4). Write a function: def solution(A) that, given a non-empty zero-indexed array A of N integers, returns the number of pairs of passing cars. The function should return −1 if the number of pairs of passing cars exceeds 1,000,000,000. For example, given: A[0] = 0 A[1] = 1 A[2] = 0 A[3] = 1 A[4] = 1 the function should return 5, as explained above. Assume that: N is an integer within the range [1..100,000]; each element of array A is an integer that can have one of the following values: 0, 1. Complexity: expected worst-case time complexity is O(N); expected worst-case space complexity is O(1), beyond input storage (not counting the storage required for input arguments).