from structures import ListNode, build_list, list_values

def merge_lists(a, b):
    dummy = ListNode()
    tail = dummy
    while a is not None and b is not None:
        if a.val <= b.val:
            tail.next, a = a, a.next
        else:
            tail.next, b = b, b.next
        tail = tail.next
    tail.next = a if a is not None else b
    return dummy.next