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.