21. 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 = []
Output: []

Example 3:

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

def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
	dummy = ListNode()
	tail = dummy

	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
	
	tail.next = list1 if list1 else list2
	return dummy.next