Test results - Codility

Tasks Details

easy

1. PassingCars

Count the number of passing cars on the road.

100%

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.

Solution

Programming language used Java 8

Time spent on task 27 minutes

Notes

not defined yet

Task timeline

Code: 09:06:36 UTC, java, verify, result: Passed

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

// you can also use imports, for example:
// import java.util.*;

// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");

class Solution {
    public static final int MAX_NUM = 1000000000;
    public int solution(int[] A) {
        // write your code in Java SE 8
        int sum = 0;
        int result = 0;
        for(int i =A.length-1; i>=0; i--){
            if(A[i] == 1){
                sum++;
            }else{
                result += sum;
                if(result>MAX_NUM) return -1;
            }
        }
        return result;
    }
}

Analysis

▶

example
example test

✔

Code: 09:06:44 UTC, java, verify, result: Passed

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

// you can also use imports, for example:
// import java.util.*;

// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");

class Solution {
    public static final int MAX_NUM = 1000000000;
    public int solution(int[] A) {
        // write your code in Java SE 8
        int sum = 0;
        int result = 0;
        for(int i =A.length-1; i>=0; i--){
            if(A[i] == 1){
                sum++;
            }else{
                result += sum;
                if(result>MAX_NUM) return -1;
            }
        }
        return result;
    }
}

Analysis

▶

example
example test

✔

Code: 09:06:49 UTC, java, final, score: 100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

// you can also use imports, for example:
// import java.util.*;

// you can write to stdout for debugging purposes, e.g.
// System.out.println("this is a debug message");

class Solution {
    public static final int MAX_NUM = 1000000000;
    public int solution(int[] A) {
        // write your code in Java SE 8
        int sum = 0;
        int result = 0;
        for(int i =A.length-1; i>=0; i--){
            if(A[i] == 1){
                sum++;
            }else{
                result += sum;
                if(result>MAX_NUM) return -1;
            }
        }
        return result;
    }
}

Analysis summary

The solution obtained perfect score.

Analysis

Detected time complexity:

O(N)

▶

example
example test

✔

▶

single
single element

✔

▶

double
two elements

✔

▶

simple
simple test

✔

▶

small_random
random, length = 100

✔

▶

small_random2
random, length = 1000

✔

▶

medium_random
random, length = ~10,000

✔

▶

large_random
random, length = ~100,000

✔

▶

large_big_answer
0..01..1, length = ~100,000

✔

▶

large_alternate
0101..01, length = ~100,000

✔

▶

large_extreme
large test with all 1s/0s, length = ~100,000

✔