How do you merge two sorted linked lists?

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

def merge_two_lists(l1, l2):
    dummy = tail = ListNode(0)
    while l1 and l2:
        if l1.val <= l2.val:
            tail.next, l1 = l1, l1.next
        else:
            tail.next, l2 = l2, l2.next
        tail = tail.next
    tail.next = l1 or l2
    return dummy.next


# --- demo ---
def build(vals):
    head = None
    for v in reversed(vals): head = ListNode(v, head)
    return head
def to_list(head):
    out = []
    while head: out.append(head.val); head = head.next
    return out

print(to_list(merge_two_lists(build([1, 4]), build([2, 5]))))   # [1, 2, 4, 5]