LinkedList

using python 2

Why LinkedList?

• model the linked based relationship directly
• versatility and flexibility: dynamically resizing array

class ListNode(object):
  def __init__(self, value, next = None): 
    self.next = next
    self.val = value
    #default value give alternative parameters

Tranverse:

• tranverse all the nodes inside a singly linked list and print their value accorfdingly to teh screen

#iteratively
def print_all_nodes(head):
  curr = head
  while curr:
    print curr.val
    curr = curr.next

#recursively
def print_all_nodes(head):
  if not head:
    return
  print head.val
  print_all_nodes(head.next)

Search:

• given a linked list and an index/ value, find the node

def search_by_index(head, index):
  if not head or index < 0
    return None
  for move_times in xrange(index):
    head = head.next
    if not head:
      return None
  return head
#python2: xrange() uses one memory a in loop, yet range() creates an list
#python 3 only has range(), using loop

Equality:

def __eq__(self, other):
 return isinstance(other, ListNode) and self.value == other.value

Add:

• a -> b -> c add f to index 1
  0 1 2 result: a->f->b->c

def add_to_index(head, index, val):
  fake_head = ListNode(None)
  fake_head.next = head
  insert_place = search_by_index(fake_head, index)
  if insert_place is None:
    return
  new_node = ListNode(val)
  insert_place.next, new_node.next = new_node, insert_place.next
  return fake_head.next

Difference between Python list and singly linked list:

	Python list	Singly linked list
access item by index	O(1)	O(n)
add/ remove head	O(n)	O(1)
add/ remove end	O(1)	O(n) find the tail + insert it
add/ remove others	O(n)	O(n)

Homework Leetcode 707, 876, 237, 203, 83, 82, 19

注意，所有要改变LinkedList本身的操作都引入dummy node

merge two linked lists

#time O(n) space O(1)
def merge(head1, head2)
  new_head = ListNode(0)
  curr_node = new_head
  while head1 is not None and head2 is not None:
    if head1.val < head2.val:
      curr_node.next = head1
      head1 = head1.next
    else:
      curr_node.next = head2
      head2 = head2.next
  if not head1:
    curr_node.next = head1
  if not head2:
    curr_node.next = head2
  return new_head.next

Double LinkedList

class DoubleListNode:
  def __init__(self, value):
    self.prev = None
    self.next = None
    self.val = value

remove

def remove_from_index(head, index)
  fake_node = DoubleListNode('Fake Node')
  fake_node.next = head
  head. prev = fake_node
  remove_node = search_by_index(head, index)
  if not remove_node:
    fake_node.next.prev = None
    return fake_node.next
  if not remove_node.prev:
    remove_node.prev.next = remove_node.next
  if not remove_node.next:
    remove_node.next.prev = remove_node.prev
  fake_node.next.prev = None
  return fake_node.next