A repository of coding challenges/tasks I came across in various job interviews. I mostly do backend programming in Go and PHP, so here you can find some solutions.

The solutions to the tasks do not fulfill to be the shortest, fastest or cleanest solutions.

Tasks

Each task should come with a solution plus some tests to verify.

Task 1

Create a function maskify to mask digits of a credit card number with #.

Requirements:

• Do not mask the first digit and the last four digits
• Do not mask non-digit chars
• Do not mask if the input is less than 6
• Return '' when input is empty

Task 2

Create a function number_to_ordinal to create an ordinal number for a given input. Ordinal numbers in English have something like st, nd, rd, etc.

Requirements:

• Apply for number 1 to 1001... if that works any given number will do ;-)

Task 3

Create a calculator for Reverse Polish Notation. Write a calculate function that accepts an input and returns the result of the operation.

Requirements:

• Support the mathematical operations for +, -, * and /
• Check for invalid syntax, like 2 3+. There is a space missing.
• Return 0 (integer) when nothing is entered
• Return the numeric value when no operand is given, like 1 2 3.5 return 3.5

Task 4

Given you have an array of numbers, you move inside the array by the value of the current element. Write a function jump_out_of_array that outputs

• the amount of jumps until you jump out of the array
• -1 when you reach the end of the array but do not jump out

Requirements:

• Array size is indefinite
• Array elements are integers, positive and negative

Example:

Given an array of A[2, 3, -1, 1, 6, 4].

• Jump 1: A[0] + 2 = A[2]
• Jump 2: A[2] + (-1) = A[1]
• Jump 3: A[1] + 3 = A[4]
• Jump 4: A[4] + 6 = out of range

So the result is 4, you need 4 jumps to jump out of the array.

Task 5

Find the k-complement pairs in an array of a given number. Write a function k_complement that that outputs the amount of pairs.

Requirements:

Do not use nested loops to solve this problem, because of a time complexity of the loop solution. Check this thread to see what time complexity of an algorithm means.

Example:

• A[0] + A[8] = 1 + 5 = 6
• A[1] + A[6] = 8 + -2 = 6
• A[4] + A[8] = 1 + 5 = 6
• A[5] + A[5] = 3 + 3 = 6
• A[5] + A[5] = 3 + 3 = 6
• A[6] + A[1] = -2 + 8 = 6
• A[8] + A[0] = 5 + 1 = 6

The result here is 7.

Task 6

Calculate the Fibonacci number of a given number and return the last 6 non-zero numbers.

Requirements:

• Use a recursive approach instead of looping through.
• Throw an exception when passing in a negative number

Example:

• F8 = 21, return 21
• F38 = 39088169, return 88169

Task 7

Given the follwing function you should suggest what could be improved. There are no other documents explaining why this function has been written or what the purpose is/should be.

Example in python

def multiply(x, y):
    if y > 0:
        return (1 + multiply(x, y-1))
    else:
        return 0

Possible considerations:

• Does the function really multiply two values?
• Could the in-built multiply function be used?
• Is a recursive function the way to go?
• What can happen when using this with big numbers, f. ex. > 1.000.000?
• Type hints

Task 8

Do an in-place mirroring of a one dimensional array. In-place switching is key here as the input array can be very big and no additional memory should be occupied - see Space Complexity.

Requirements:

• In-place mirroring
• Handle array with even and odd amount of items
• Do not use the array_reverse function in PHP

Example:

• Even amount: [8,5,1,4] -> [4,1,5,8]
• Odd amount: [6,2,7,9,3] -> [3,9,7,2,6]

Contribute

Feel free to contribute. Use the issue list to propose new tasks or open PRs. Just provide proper tests and description and requirements for the tasks.