leetcode 21 - merge two sorted lists.py

"""
You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Question: https://leetcode.com/problems/merge-two-sorted-lists/

"""


class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        answer = ListNode()
        tail = answer
        while list1 and list2:
            if list1.val < list2.val:
                tail.next = list1
                list1 = list1.next
            else:
                tail.next = list2
                list2=list2.next
            tail = tail.next
            
        if list1:
                tail.next = list1
        elif list2:
                tail.next = list2
                
        return answer.next