## среда, 25 декабря 2013 г.

### Codility. Train. Passing-cars ★★

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
A = 1
A = 0
A = 1
A = 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 passing cars.
The function should return −1 if the number of passing cars exceeds 1,000,000,000.
For example, given:
```  A = 0
A = 1
A = 0
A = 1
A = 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 within the range [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).
Elements of input arrays can be modified.

Solution:

```def solution(A):
c, z = 0, 0
for i in A:
if i == 0:
z += 1
else:
c += z
if c > 1000000000:
return -1
return c
```