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;
}