11 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.

The function should return −1 if the number of pairs of passing cars exceeds 1,000,000,000.
------------------------------------------------------------------------------*/


function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    let east = 0;
    let maxPassingCars = 1000000000;
    let passingCars = 0;
    
    A.forEach(function (car) {
        if (car === 0) east += 1 
        if (car === 1) {
            passingCars += east;
            if (passingCars > maxPassingCars) return -1;
        }
    });
    
    return passingCars;
}