Bad performance in Java exercise

Question

I'm doing some tests in Java to warm up, and I just made this one:

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:

class Solution { public int solution(int[] A); }

that, given a non-empty zero-indexed 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.

Assume that:

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. 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.

My code is as follows:

public int solution(int[] A) {
        // write your code in Java SE 8
        int result = 0;
        long mult = 0;

        for(int value : A){

            if(value == 0){
                mult ++;        
            }
            else {
                result += mult;
            }
        }

        return result;
    }

The link with the result is this one: https://codility.com/demo/results/trainingFFF4BS-AZ3/

If the link die, the result said:

Performance tests

▶ medium_random random, length = ~10,000 ✔OK ▶ large_random random, length = ~100,000 ✘WRONG ANSWER got 1248768710 expected -1 ▶ large_big_answer 0..01..1, length = ~100,000 ✘WRONG ANSWER got -1794967296 expected -1 ▶ large_alternate 0101..01, length = ~100,000 ✘WRONG ANSWER got 1250025000 expected -1 ▶ large_extreme large test with all 1s/0s, length = ~100,000 ✔OK

Any ideas what is wrong in my code?.

Answer 1

Your rules say,

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

And you don't test that condition. You could with a ternary operation and something like

return result > 1000000000 ? -1 : result;

or (debateably) more readable

if (result > 1000000000) {
    return -1;
}
return result;

and for improved performance you might add that test like

for (int value : A) {
    if (value == 0) {
        mult++;
    } else {
        result += mult;
        if (result > 1000000000) {
            return -1;
        }
    }
}
return result;