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Merge Two Sorted Lists

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

Merge the two lists into 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.

Example 1

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2

Input: list1 = [], list2 = [0]
Output: [0]

Solution Explanation

This problem just requires that you don't don't traverse through entire linked list when one of the linked list already has been traversed. It's already sorted so what we can do is, we can just traverse through until we reach the end of shorted list, then just join the remaining other list.
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
# = nex
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
      if not list1: return list2
      if not list2: return list1

      dummy = ListNode()
      current = dummy

      while list1 and list2:
        if list1.val < list2.val:
 = list1
          list1 =
 = list2
          list2 =
        current =
      if list1: = list1
      if list2: = list2