List<SinglyLinkedListNode> l = new ArrayList<>();
while (head1 != null || head2 != null) {
	if (head1 != null) {
		l.add(head1);
	}
	if (head2 != null) {
		l.add(head2);
	}
	head1 = head1 != null ? head1.next : null;
	head2 = head2 != null ? head2.next : null;
}
l = l.stream()
	.sorted(Comparator.comparingInt(n -> n.data))
	.collect(java.util.stream.Collectors.toList());
for (int i = 0; i < l.size() - 1; i++) {
	SinglyLinkedListNode c = l.get(i);
	c.next = l.get(i+1);
}
return l.get(0);