21. Merge Two Sorted Lists - LeetCode

from __future__ import annotations


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


class Solution:
    def mergeTwoLists(self, list1: ListNode | None, list2: ListNode | None) -> ListNode | None:
        # if (list1 is None) and (list2 is None):
        #     return None

        # if (list1 is None) or (list2 is None):
        #     return list1 or list2

        p = dummy = ListNode()

        p1, p2 = list1, list2
        while p1 and p2:
            if p1.val < p2.val:
                p.next, p1 = p1, p1.next
            else:
                p.next, p2 = p2, p2.next
            p = p.next
        p.next = p1 or p2
        return dummy.next