Implement a method insertfirst which takes a linked list as


Python 3 Linked List Insert Element Problem

Implement a method, insert_first which takes a linked list as a parameter and elem, and returns a new list, where elem was inserted as a first node. 

def insert_first(s, elem):

  '''

  >>> insert_first(link(3, link(2, link(1, "empty"))),100)

  [100, [3, [2, [1, 'empty']]]]

  '''

Implement a method, insert_last which takes a linked list as a parameter and elem, and returns a new list, where elem was inserted as a last node. 

def insert_last(s, elem):

  '''

  >>> lst = link(2, link(1, empty))

  >>> insert_last(lst,100)

  [2, [1, [100, 'empty']]]

  '''

Implement insert_at which takes a linked-list, an element elem and an insertion index idx and returns the list with the element inserted at the correct location.

def insert_at(s, elem, idx):

  '''

  >>> lst = link(3, link(2, link(1, empty)))

  >>> insert_at(lst, 4, 0)

  [4, [3, [2, [1, 'empty']]]]

  >>> insert_at(lst, 4, 2)

  [3, [2, [4, [1, 'empty']]]]

  '''

IMPORTANT Information:

empty = 'empty'

def is_link(s):

  """s is a linked list if it is empty or a (first, rest) pair."""

  return s == empty or (len(s) == 2 and is_link(s[1]))

def link(first, rest):

  """Construct a linked list from its first element and the rest."""

  assert is_link(rest), "rest must be a linked list."

  return [first, rest]

def first(s):

  """Return the first element of a linked list s."""

  assert is_link(s), "first only applies to linked lists."

  assert s != empty, "empty linked list has no first element."

  return s[0]

def rest(s):

  """Return the rest of the elements of a linked list s."""

  assert is_link(s), "rest only applies to linked lists."

  assert s != empty, "empty linked list has no rest."

  return s[1]

# define length

def len_link(s):

  """Return the length of linked list s."""

  length = 0

  while s != empty:

    s, length = rest(s), length + 1

  return length

# getitem

def getitem(s, i):

  """Return the element at index i of linked list s."""

  while i > 0:

    s, i = rest(s), i - 1

  return first(s)

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Basic Computer Science: Implement a method insertfirst which takes a linked list as
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