Count Distinct Elements in Every Window

The problem. Given an array arr and a number k, return how many distinct numbers each window of k consecutive elements contains — one count per window, from left to right. An array of n elements has n − k + 1 windows.

Example

arr = [1, 2, 1, 3, 4, 2, 3],  k = 4  ->  [3, 4, 4, 3]
arr = [4, 1, 1],              k = 2  ->  [2, 1]
arr = [5, 5, 5, 5],           k = 3  ->  [1, 1]