我的链表实现的成员关系检查方法中的Bug

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

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, value):
        if not self.head:
            self.head = ListNode(value)
        else:
            current = self.head
            while current.next:
                current = current.next
            current.next = ListNode(value)

    def contains(self, target):
        current = self.head
        while current:
            if current.value == target:
                return True
            current = current.next
        return False

# 示例使用
linked_list = LinkedList()
linked_list.append(1)
linked_list.append(2)
linked_list.append(3)

print(linked_list.contains(2))  # 输出: True
print(linked_list.contains(4))  # 输出: False