swap_node_pairs.py

def swap_node_pairs(head):

    if not head or not head.next:
        return head

    dummy_node = ListNode()
    dummy_node.next = head
    prev = dummy_node
    i = head
    j = head.next

    while prev.next and prev.next.next:
        # finish here
        j = i.next
        # code logically starts from here
        i.next = j.next
        j.next = i
        prev.next = j

        prev = j.next
        i = i.next

    return dummy_node.next