Method convert

java/Chapter 17/Question17_13/QuestionA.java:14–31

(BiNode root)

	}
	
	public static NodePair convert(BiNode root) {
		if (root == null) {
			return null;
		}
		
		NodePair part1 = convert(root.node1);
		NodePair part2 = convert(root.node2);
		
		if (part1 != null) {
			concat(part1.tail, root);
		}
		
		if (part2 != null) {
			concat(root, part2.head);
		}
		
		return new NodePair(part1 == null ? root : part1.head, part2 == null ? root : part2.tail);
	}	
	
	public static void concat(BiNode x, BiNode y) {
		x.node2 = y;