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] = 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 passing cars.
The function should return −1 if the number 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 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

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