Codility Test (11. PassingCars)

Codility Test (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.

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:

class Solution { public int solution(int[] 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.

問題回答

class Solution {
    public int solution(int[] A) {
        // write your code in Java SE 8
        int count = 0;
        int rcount = 0;
        for(int i = 0; i < A.length; i++) {
            if(A[i] == 0) {
                count++;
            }
            if(A[i] == 1) {
                rcount += count;
            }
            if(rcount > 1000000000) {
                return -1;
            }
        }
        return rcount;
    }
}