ListNode is not defined 怎么办

这个错误通常表示您正在使用 `ListNode`，但它并没有被定义。在使用 `ListNode` 之前，您需要定义它。

如果您是在使用 Python，则需要自己实现一个链表节点类。例如：

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

如果您使用的是其他语言，可能需要查看您正在使用的库或框架中是否包含定义 `ListNode` 的文件或类。

相关问题

ListNode is not defined

`ListNode` is a class that is commonly used in the context of linked lists in programming languages such as Python and Java. It is possible that you are trying to use `ListNode` without defining it first.

To use `ListNode`, you will need to define it in your code. Here's an example of how you can define `ListNode` in Python:

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

This code defines a `ListNode` class with two attributes: `val` and `next`. `val` stores the value of the node, while `next` stores a reference to the next node in the linked list.

Once you have defined `ListNode`, you can create instances of it and use it to build linked lists.

ListNode' object is not subscriptable

This error occurs when you try to access an element of a ListNode object using the square bracket notation, but the object does not support subscripting.

For example, if you have a ListNode object like this:

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

And you try to access an element using the square bracket notation like this:

node = ListNode(1)
print(node[0])

You will get the error "TypeError: 'ListNode' object is not subscriptable" because ListNode does not support subscripting.

To access the values of a ListNode object, you can use a loop to iterate over the nodes:

node = ListNode(1)
while node is not None:
    print(node.val)
    node = node.next