Here's my Java 8 solution, passing all tests, for Max Score of 35:

Code is fairly simple & self-explanatory. First sort the List. Then iterate through it once, doing a "sliding sublist window", calculate Current Diff by considering the first / last element of the sub-list as Min / Max, and compare that with the "Overall Min Diff" & update as needed.

`

public static int maxMin(int k, List<Integer> arr) {
    // Write your code here
    int minUnfairness = Integer.MAX_VALUE;

    Collections.sort(arr);

    for (int i = 0 ; i <= (arr.size() - k) ; i++) {
        List<Integer> curSubLst = arr.subList(i, (i + k));
        int curUnfairness = curSubLst.get(curSubLst.size() - 1) - curSubLst.get(0);
        minUnfairness = Math.min(minUnfairness, curUnfairness);
    }

    return minUnfairness;
}

`

This problem's solution approach is very similar / identical to the "Minimum Absolute Difference in an Array" problem: as explained in this comment I have posted in the discussions of that problem.

Here is my Python solution!

def maxMin(k, arr):
    arr.sort()
    unfairness = arr[k - 1] - arr[0]
    for i in range(1, len(arr) - k + 1):
        unfairness = min(arr[k + i - 1] - arr[i], unfairness)
    return unfairness

Java:

public static int maxMin(int k, List arr) {

    int[] arr2 = arr.stream().mapToInt(i -> i).toArray();

    //primitive arrays are faster

    Arrays.sort(arr2);

    int diff = Integer.MAX_VALUE;

    for (int j=0; j < arr2.length - k + 1; ++j) {

        if (arr2[j+k-1] - arr2[j] < diff) {

            diff = arr2[j+k-1] - arr2[j];

        }

    }
    return diff;
}

def maxMin(k, arr): return min(map(lambda x, y: abs(x - y), sorted(arr)[:-k + 1], sorted(arr)[k - 1:]))

JS

function maxMin(k, arr) {
  // sort - lowest to highest
  arr.sort((a, b) => a - b);

  // calculate unfairness of "first chunk" of size k
  let minUnfairness = arr[k - 1] - arr[0];

  for (let i = 1; i + k - 1 < arr.length; i++) {
    // calculate unfairness in "next chunk" of size k
    const unfairness = arr[i + k - 1] - arr[i];

    if (unfairness < minUnfairness) {
      minUnfairness = unfairness;
    }
  }

  return minUnfairness;
}