N26 Backend Engineer interview questions

(1) Codity questions (3 coding problems described below) : (1) You are given implementation of a function . This function, given a non-empty array A of N integers (sorted in non-decreasing order) and integer K, checks whether A contains numbers 1,2, ..., K (every number from 1 to K atleast once) and no other numbers. The goal of the exercise is to find and fix bug(s) in the implementation. (2) In 1 step, any element of a given array can be either increased or decreased by 1. Write a function that, given an array A of N integers, returns the minimum number of steps required to make all elements equal. (3) There are N arrows. The K-th arrow is rotated clockwise by A[K] degrees from an upward direction. In 1 move you can rotate 1 arrow clockwise or counter-clockwise by an arbitrary angle. What is the minimum sum of rotation angles needed to make all arrows point in the same direction. Write a function that, given an array A made of N integers, describing the initial rotation of each arrow, returns the minimum sum of rotation angles needed to make all arrows point in the same direction. (2) Coding interview: (1) code Leetcode easy level question. (3 Sum). (2) If a client sends invalid or unexpected input to a REST API, what should the API ideally return? (3) What are sealed interfaces? When / why do we use it? (4) if a rest api needs to return null to the client, should it return a simple null or return an object of type Optional ? When to use Optional? What are the benefits of using Optional? If the type of input from the client is not as expected, would you prefer to return a null, or an error message or an empty collection? (5) If a variable is of type Optional, do we still need to perform a null check before calling methods on it? What happens if we don't? Error or warning? (3) Design interview (1) Design a new user registration system, including the email verification/confirmation flow.