code: import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  
  class Hailstone {
  
    public static List<Long> getHailstoneSequence(long n) {
      if (n <= 0)
        throw new IllegalArgumentException("Invalid starting sequence number");
      List<Long> list = new ArrayList<Long>();
      list.add(Long.valueOf(n));
      while (n != 1) {
        if ((n & 1) == 0)
          n = n / 2;
        else
          n = 3 * n + 1;
        list.add(Long.valueOf(n));
      }
      return list;
    }
  
    public static void main(String[] args) {
      List<Long> sequence27 = getHailstoneSequence(27);
      System.out.println("Sequence for 27 has " + sequence27.size() + " elements: " + sequence27);
  
      long MAX = 100000;
      // Simple way
      {
        long highestNumber = 1;
        int highestCount = 1;
        for (long i = 2; i < MAX; i++) {
          int count = getHailstoneSequence(i).size();
          if (count > highestCount) {
            highestCount = count;
            highestNumber = i;
          }
        }
        System.out.println("Method 1, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
      }
  
      // More memory efficient way
      {
        long highestNumber = 1;
        int highestCount = 1;
        for (long i = 2; i < MAX; i++) {
          int count = 1;
          long n = i;
          while (n != 1) {
            if ((n & 1) == 0)
              n = n / 2;
            else
              n = 3 * n + 1;
            count++;
          }
          if (count > highestCount) {
            highestCount = count;
            highestNumber = i;
          }
        }
        System.out.println("Method 2, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
      }
  
      // Efficient for analyzing all sequences
      {
        long highestNumber = 1;
        long highestCount = 1;
        Map<Long, Integer> sequenceMap = new HashMap<Long, Integer>();
        sequenceMap.put(Long.valueOf(1), Integer.valueOf(1));
  
        List<Long> currentList = new ArrayList<Long>();
        for (long i = 2; i < MAX; i++) {
          currentList.clear();
          Long n = Long.valueOf(i);
          Integer count = null;
          while ((count = sequenceMap.get(n)) == null) {
            currentList.add(n);
            long nValue = n.longValue();
            if ((nValue & 1) == 0)
              n = Long.valueOf(nValue / 2);
            else
              n = Long.valueOf(3 * nValue + 1);
          }
          int curCount = count.intValue();
          for (int j = currentList.size() - 1; j >= 0; j--)
            sequenceMap.put(currentList.get(j), Integer.valueOf(++curCount));
          if (curCount > highestCount) {
            highestCount = curCount;
            highestNumber = i;
          }
        }
        System.out.println("Method 3, number " + highestNumber + " has the longest sequence, with a length of " + highestCount);
      }
      return;
    }
  }
  