Optional[ListNode] is a type hint in Python that indicates that a function or variable can have a value of either ListNode or None. It is used to provide type information to static type checkers and linters, and can also make it easier for developers to understand the code.
In this case, it is likely that the Optional[ListNode] type hint is being used to indicate that a function or variable can be either a ListNode object or None, possibly to represent the end of a linked list.
Type hints are a feature of Python that allow you to annotate variables and function signatures with the expected type of the value. They are not enforced by the Python interpreter, but can be used by static type checkers and linters to catch type errors and improve code readability.
For example, you might use Optional[ListNode] as the return type hint for a function that returns a ListNode object or None, like this:
def get_next_node(node: ListNode) -> Optional[ListNode]: if node is None: return None return node.next
In this example, the get_next_node() function takes a ListNode object as an argument and returns either another ListNode object or None, depending on whether the input node is None. The Optional[ListNode] type hint indicates that the return value can be either a ListNode or None.
For example, consider the following code:
from typing import Optional def find_node(node: Optional[ListNode], value: int) -> Optional[ListNode]: if node is None: return None if node.value == value: return node return find_node(node.next, value)
In the above code, the find_node function takes a node of type Optional[ListNode] and a value of type int, and it returns a value of type Optional[ListNode]. This means that the node argument can be either a ListNode object or None, and the return value can be either a ListNode object or None.
Using the Optional type in this way allows you to write functions that can handle None values more explicitly, making the code easier to read and understand.