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