Solutions

# Use a dummy head, then iterate through both lists, appending the smaller node to the merged list. Finally, append any remaining nodes from either list.
# linked-lists

from typing import Optional

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

# O(n + m) time | O(1) space
def mergeTwoLists(list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
    newhead = ListNode(-1, None)
    p1, p2 = list1, list2
    curr = newhead
    while p1 and p2:
        if p1.val < p2.val:
            curr.next = p1
            p1 = p1.next
        else:
            curr.next = p2
            p2 = p2.next
        curr = curr.next

    if p1:
        curr.next = p1
    elif p2:
        curr.next = p2

    return newhead.next