Python链表合并排序不起作用

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
        
def mergeTwoLists(l1, l2):
    if not l1:
        return l2
    if not l2:
        return l1
    if l1.val < l2.val:
        l1.next = mergeTwoLists(l1.next, l2)
        return l1
    else:
        l2.next = mergeTwoLists(l1, l2.next)
        return l2

# 示例用法
l1 = ListNode(1)
l1.next = ListNode(3)
l2 = ListNode(2)
l2.next = ListNode(4)
merged_list = mergeTwoLists(l1, l2)

def mergeTwoLists(l1, l2):
    dummy = ListNode(0)  # 创建一个哑节点作为新链表的头节点
    current = dummy  # 当前节点指针

    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

    current.next = l1 if l1 else l2  # 处理剩余的节点

    return dummy.next

# 示例用法
l1 = ListNode(1)
l1.next = ListNode(3)
l2 = ListNode(2)
l2.next = ListNode(4)
merged_list = mergeTwoLists(l1, l2)