//The idea behind this function is to scan the tree "in order" recursively
   public void recKeysToArray(IWAVLNode currNode, int[] arr) {
	   if(!currNode.isRealNode()) //If currNode is a virtual node, do nothing
		   return;
	   if(((WAVLNode)currNode).getRank() == 0) { //If currNode is a leaf
		   arr[currIndex] = currNode.getKey();
		   currIndex++;
	   }
	   else {
		   recKeysToArray(currNode.getLeft(), arr);
		   arr[currIndex] = currNode.getKey();
		   currIndex++;
		   recKeysToArray(currNode.getRight(), arr);
	   }
   }
  /**
   * public int[] keysToArray()
   *
   * Returns a sorted array which contains all keys in the tree,
   * or an empty array if the tree is empty.
   */
  public int[] keysToArray()
  {
	  this.currIndex = 0;
      int[] arr = new int[this.root.getSubtreeSize()]; //num of keys = num of nodes in the tree
      recKeysToArray(this.root, arr);
      return arr;
  }