420.java

public int periodLength(int[] heaps) {
    ArrayList<Integer> slow = new ArrayList<Integer>();
    for (int i=0; i<heaps.length; i++) slow.add(heaps[i]);
    Collections.sort(slow);
    ArrayList<Integer> fast = slow;

    while (true) {
      slow = step(slow);
      fast = step(fast);
      fast = step(fast);
      if (slow.equals(fast)) {
        int period = 0;
        while (true) {
          slow = step(slow);
          period++;
          if (slow.equals(fast)) return period;
        }
      }
    }
  }