PassingCars

A non-empty 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:

function solution($A);

that, given a non-empty 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.

Write an efficient algorithm for the following assumptions:

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.


******************************************** SOLUTION ********************************************

function solution($A) {
    $eastCars = 0;
    $passing = 0;  
    
    $length = sizeof($A);
    
    for ($i = 0; $i < $length; $i++) {
        
        if ($A[$i] == 1) {
            $passing += $eastCars;
        } else {
            $eastCars++;
        }
        
        if ($passing > 1000000000) {
            return -1;
        }
    }
    
    return $passing;
}