在python中合并2个链接表不起作用。创建的第三个链表在执行后返回空值/结果

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def mergeTwoLists(l1, l2):
    # 创建一个空的头节点
    dummy = ListNode()
    # 创建一个指针用于遍历链表
    current = dummy

    # 遍历两个链表
    while l1 and l2:
        if l1.val < l2.val:
            current.next = l1
            l1 = l1.next
        else:
            current.next = l2
            l2 = l2.next
        current = current.next

    # 如果其中一个链表为空，直接将另一个链表连接到结果链表的末尾
    current.next = l1 if l1 else l2

    # 返回结果链表的头节点
    return dummy.next

# 创建链表
# l1: 1 -> 2 -> 4
l1 = ListNode(1)
l1.next = ListNode(2)
l1.next.next = ListNode(4)

# l2: 1 -> 3 -> 4
l2 = ListNode(1)
l2.next = ListNode(3)
l2.next.next = ListNode(4)

# 合并链表
result = mergeTwoLists(l1, l2)

# 打印合并后的链表结果
while result:
    print(result.val)
    result = result.next