It’s been a while since I took my last challenge on Codility and this was the first one I did in 2019. I felt a little bit rusty but it was a great weekend algorithm challenge. I managed to do 100% points on correctness and performance which is awarded with a Golden Award.

The challenge was marked as hard but personally I didn’t felt like it was on the level of the hardest challenges on Codility.

If you want to know more details about the challenge here is the problem for which you need to find a solution for.

*Integers K, M and a non-empty array A consisting of N integers, not bigger than M, are given.*

*The leader of the array is a value that occurs in more than half of the elements of the array, and the segment of the array is a sequence of consecutive elements of the array.*

*You can modify A by choosing exactly one segment of length K and increasing by 1 every element within that segment.*

*The goal is to find all of the numbers that may become a leader after performing exactly one array modification as described above.*

*Write a function:*

*class Solution { public int[] solution(int K, int M, int[] A); }*

*that, given integers K and M and an array A consisting of N integers, returns an array of all numbers that can become a leader, after increasing by 1 every element of exactly one segment of A of length K. The returned array should be sorted in ascending order, and if there is no number that can become a leader, you should return an empty array. Moreover, if there are multiple ways of choosing a segment to turn some number into a leader, then this particular number should appear in an output array only once.*

*For example, given integers K = 3, M = 5 and the following array A:*

*A[0] = 2*

*A[1] = 1*

*A[2] = 3*

*A[3] = 1*

*A[4] = 2*

*A[5] = 2*

*A[6] = 3*

*the function should return [2, 3]. If we choose segment A[1], A[2], A[3] then we get the following array A:*

*A[0] = 2*

*A[1] = 2*

*A[2] = 4*

*A[3] = 2*

*A[4] = 2*

*A[5] = 2*

*A[6] = 3*

*and 2 is the leader of this array. If we choose A[3], A[4], A[5] then A will appear as follows:*

*A[0] = 2*

*A[1] = 1*

*A[2] = 3*

*A[3] = 2*

*A[4] = 3*

*A[5] = 3*

*A[6] = 3*

*and 3 will be the leader.*

*And, for example, given integers K = 4, M = 2 and the following array:*

*A[0] = 1*

*A[1] = 2*

*A[2] = 2*

*A[3] = 1*

*A[4] = 2*

*the function should return [2, 3], because choosing a segment A[0], A[1], A[2], A[3] and A[1], A[2], A[3], A[4] turns 2 and 3 into the leaders, respectively.*

*Write an efficient algorithm for the following assumptions:*

*N and M are integers within the range [1..100,000];*

*K is an integer within the range [1..N];*

*each element of array A is an integer within the range [1..M].*

So try it yourself and I hope you’ll have some nerd fun as I did. Just go on this link https://app.codility.com/programmers/challenges/.

Happy coding,

J.