Leetcode 2122. Recover the Original Array

Given a multiset of 2n numbers that are exactly the values arr[i]-k and arr[i]+k for some unknown positive k, recover arr by finding a positive k such that the numbers can be paired into n couples with difference 2k and taking each arr[i] as the midpoint of its pair — typically done by sorting and greedy multiset/counter pairing using candidate differences from the smallest element.