code: public ArrayList<Integer> getUnvisited() {
      if (0 == this.unvisitedCount) {
        return new ArrayList<Integer>();
      }
      ArrayList<Integer> res = new ArrayList<Integer>(this.unvisitedCount);
      for (int i = 0; i < this.registry.length; i++) {
        if (ZERO == this.registry[i]) {
          res.add(i);
        }
      }
      return res;
    }
docs: Get the list of unvisited positions.
  
  @return list of unvisited positions.