merge_tsl.py

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

class Solution:
    def mergeTwoLists(self, list1, list2):
        if list1 and not list2:
            return list1

        if list2 and not list1:
            return list2

        if not list1 and not list2:
            return None

        dummy = ListNode()
        curr = dummy

        while list1 and list2:
            if list1.val < list2.val:
                curr.next = list1
                curr = list1
                list1 = list1.next
            else:
                curr.next = list2
                curr = list2
                list2 = list2.next

        curr.next = list1 if list1 else list2

        return dummy.next