Count the number of passing cars on the road.

PassingCars.java

public class PassingCars {
	public int solution(int[] A) {
		// 0 - east, 1 - west
		int west = 0;
		int passing = 0;
		
		for(int i = A.length - 1; i >= 0; i--) {
			if(A[i] == 0) {
				passing += west;
				if(passing > 1000000000) {
					return -1;
				}
			} else {
				west += 1;
			}
		}
		
		return passing;
	}
}