import java.util.*;

class Solution {
  /**
   * Sorts a list of Dutch mobile phone numbers using LSD radix sort.
   *
   * @param phoneNumbers
   *     The list of phone numbers to sort.
   * @return The sorted list of phone numbers.
   * @throws NullPointerException
   *     If `phoneNumbers` equals `null`.
   */
  static List<String> radixSortLSD(List<String> phoneNumbers) {
   if (phoneNumbers == null){
     throw new NullPointerException();
   }

   ArrayList<String> list = new ArrayList<>();
    list.addAll(phoneNumbers);

   Queue<String>[] buckets = new LinkedList[10];
   for (int i = 0; i<buckets.length; i++){
     buckets[i] = new LinkedList<String>();
   }


   for (int j = 9; j>=2; j--){
      for (int i = 0; i<list.size(); i++){
    buckets[Character.getNumericValue(list.get(i).charAt(j))].add(list.get(i));
    }
    list.clear();
      for (int i=0; i<buckets.length; i++){
        list.addAll(buckets[i]);
        buckets[i].clear();
   }
   }
   return list;
  }
}