def mergeLists(head1, head2):
    
    dummy = SinglyLinkedListNode(0)
    current = dummy
    
    while head1 and head2:
        
        if head1.data <= head2.data:
            current.next = head1
            head1 = head1.next
        else:
            current.next = head2
            head2 = head2.next
            
        current = current.next

    if head1:
        current.next = head1
    if head2:
        current.next = head2

    return dummy.next