尝试将两个链表添加到一起，但获得了AttributeError

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

def merge_lists(l1, l2):
    dummy = ListNode(0)
    curr = dummy

    while l1 and l2:
        if l1.val < l2.val:
            curr.next = l1
            l1 = l1.next
        else:
            curr.next = l2
            l2 = l2.next
        curr = curr.next

    if l1:
        curr.next = l1
    if l2:
        curr.next = l2

    return dummy.next

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

# 创建链表2: 1 -> 3 -> 4
l2 = ListNode(1)
l2.next = ListNode(3)
l2.next.next = ListNode(4)

# 合并链表
merged_list = merge_lists(l1, l2)

# 打印合并后的链表: 1 -> 1 -> 2 -> 3 -> 4 -> 4
while merged_list:
    print(merged_list.val, end=" -> ")
    merged_list = merged_list.next
print("None")