CS 261

1
CS 261 Data Structures

• Linked Lists Introduction
• Implementation of Stack

2
Linked Lists Characteristics

• Data elements held in structures called links
• Like a chain each link is tied to the next
• Links are 1 1 with elements, allocated and
  released as necessary
• Each link points to next link in sequence,
  sometimes to previous link
• Lots of variations on a simple idea

link
link
data
data

next link
next link
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Typical Link Structure Single Linked List

• struct Link / Single link. /
• TYPE val / Data contained by this
  link. /
• struct Link next / Pointer to next link. /

next
val
4
Linked List Variations

• All linked lists consists of links but there
  are other design decisions
• Header (special value to point to start) or no
  header?
• Use null as terminator, or special value
  (sentinel) for end?
• Use single or double links?
• Pointer to first element only, or pointer to
  first and last?

5
Link List Stack Simplest List Structure

• Implementing a stack interface with a linked
  list
• Header with head reference only null if empty
• No sentinel null terminated
• Singly linked
• Elements added or removed from front
• Only access first element

Very Simple
6
List Stack Illustration
head
val 2 next
val 7 next
val 4 next null
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List Stack Implementation

• struct ListStack
• struct Link head / Initialize routine sets
  to zero/NULL. /
• void pushListStack(struct ListStack s, double
  val)
• / You are going to write this
• 1. Allocate (malloc) a new link (check that it
  works!).
• 2. Set data fields in the new link.
• 3. Change head to point to new link. /

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Other Operations?

• How do you tell if stack is empty?
• How do you return first element (i.e., head)?
• How do you remove an element?

9
Linked List Stack Time Complexity

• Time complexity of ListStack operations
• Push O(1) always
• Pop O(1) always
• Top O(1) always
• How would this compare to a DynArr (a dynamic
  array implementation of a stack)?
• Push O( )
• Pop O( )
• Top O( )
• In practice, dynamic array is slightly faster in
  real timings

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List Stack Computational Complexity

• Compare linked list stack to dynamic array
  stack
• In practice dynArr is slightly faster in real
  timings

ListStack DynArr
push O(1) always O(1) expected
pop O(1) always O(1) always
top O(1) always O(1) always
11
Your turn

• Complete worksheet on linked list stack
• Next time New ADT topic ? Queues
• Read the lesson on queues posted on web site