How to represent these singly linked list nodes in the leetcode test runner in a Python script?

I am struggling with these linked list problems on leetcode (and the tree problems since they use a similar structure). Example https://leetcode.com/problems/merge-two-sorted-lists/

If I try to run this code outside of leetcode’s magic box, I run into issues.

My mergeTwoLists function works great inside of the leetcode editor (runs successfully, is accepted)

from typing import Optional


# Definition for singly-linked list.
class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


def mergeTwoLists(list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
    prehead = ListNode(-1)

    prev = prehead
    while list1 and list2:
        if list1.val <= list2.val:
            prev.next = list1
            list1 = list1.next
        else:
            prev.next = list2
            list2 = list2.next
        prev = prev.next
        breakpoint()

    prev.next = list1 if list1 is not None else list2

    return prehead.next


if __name__ == '__main__':
    l1 = ListNode(1)
    l1_2 = ListNode(2)
    l1_3 = ListNode(4)
    l1.next = l1_2
    l1_2.next = l1_3

    l2 = ListNode(1)
    l2_2 = ListNode(3)
    l2_3 = ListNode(4)
    l2.next = l1_2
    l2_2.next = l1_3
    print(mergeTwoLists(l1, l2))

The problem is that when I try to run the above standalone from my terminal it gets into an infinite loop.

The reason is that due to what I have passed into the function, if I throw a breakpoint() into the end of the first loop and inspect list1 and list2

(Pdb) list1
<__main__.ListNode object at 0x00000294E1F8FEE0>
(Pdb) list2
<__main__.ListNode object at 0x00000294E1F8FE20>

Which makes sense. It explains the infinite loop, class objects eval to True when cast to bool.

So based on this, if I want to get this code to run outside leetcode I need to do something differently. I need more info for my class definition, or my test case setup is wrong?

Does anyone know more about the complete definition of the ListNode class in Leetcode’s system? Or is there something I am missing in how I should be setting up my test?

Thank you!

>Solution :

So yes, you are correct, on your while list1 and list2 this evaluates to True.

You can solve it like this, being more explicit:

def mergeTwoLists(list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
    prehead = ListNode(-1)

    prev = prehead
    while (list1.next is not None) and (list2.next is not None):
        if list1.val <= list2.val:
            prev.next = list1
            list1 = list1.next
        else:
            prev.next = list2
            list2 = list2.next
        prev = prev.next

    prev.next = list1 if list1 is not None else list2

    return prehead.next

It probably works in leetcode because they defined another method to the class. If you define the class like this:

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
    def __bool__(self):
        """Return bool(self)."""
        return self.next is not None

Then the __bool__ method will be called when you try to do list1 and list2, and it does the same behavior.