4.3 Stacks, Queues, and Linked Lists

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1 4.3 Stacks, Queues, and Linked Lists Introduction to Programming in Java: An Interdisciplinary Approach Robert Sedgewick and Kevin Wayne Copyright /21/2013 8:59:11 PM

2 Data Types and Data Structures Data types. Set of values and operations on those values. Some are built into the Java language: int, double[], String, Most are not: Complex, Picture, Stack, Queue, ST, Graph, Data structures. Represent data or relationships among data. Some are built into Java language: arrays. Most are not: linked list, circular list, tree, sparse array, graph, this lecture 2

Collections Fundamental data types. Set of operations (add, remove, test if empty) on generic data.

[LIFO = last in out] Remove the item most recently added. Ex: Pez, cafeteria trays, Web surfing.

[FIFO = in, out] Remove the item least recently added. Ex: Guitar Hero (RingBuffer) Symbol table.

3 Collections Fundamental data types. Set of operations (add, remove, test if empty) on generic data. Intent is clear when we insert. Which item do we remove? Stack. [LIFO = last in out] Remove the item most recently added. Ex: Pez, cafeteria trays, Web surfing. Queue. [FIFO = in, out] Remove the item least recently added. Ex: Guitar Hero (RingBuffer) Symbol table. Remove the item with a given key. Ex: Phone book

4 Linked Lists 11

5 Sequential vs. Linked Allocation Sequential allocation. Put items one after another. TOY: consecutive memory cells. Java: array of objects. Linked allocation. Include in each object a link to the next one. TOY: link is memory address of next item. Java: link is reference to next item. addr value B0 "Alice" addr C0 value "Carol" B1 "Bob" C1 null B2 "Carol" C2 Key distinctions. get i th item Array: random access, fixed size. B3 B4 B5 C3 C4 C5 "Alice" CA Linked list: sequential access, variable size. B6 B7 C6 C7 get next item B8 C8 B9 C9 BA CA "Bob" BB CB C0 array (B0) linked list (C4) 12

6 SinglyLinked Data Structures From the point of view of a particular object: all of these structures look the same. sequential (this lecture) circular parentlink tree rho general case Multiplylinked data structures. Many more possibilities. 13

7 Linked Lists Linked list. A recursive data structure. An item plus a pointer to another linked list (or empty list). Unwind recursion: linked list is a sequence of items. Node data type. A reference to a String. A reference to another Node. public class Node { public String item; public Node next; } Alice Bob Carol null item next special pointer value null terminates list 14

8 Building a Linked List Node third = new Node(); third.item = "Carol"; third.next = null; addr C0 C1 Value "Carol" null Node second = new Node(); second.item = "Bob"; second.next = third; Node = new Node();.item = "Alice";.next = second; second third C4 CA C0 C2 C3 C4 C5 C6 "Alice" CA C7 C8 C9 CA "Bob" CB C0 second third CC CD Alice Bob Carol item next null CE CF main memory 15

9 Stack Push (Insert at Front): Linked List Implementation best the was it second best the was it Node second = ; second best the was it = new Node(); of second best the was it.item = "of";.next = second; 16

10 Stack Pop (Remove from Front): Linked List Implementation "of" of best the was it String item =.item; of best the was it =.next; garbagecollected best the was it return item; 17

Stack: Linked List Implementation public class LinkedStackOfStrings { private Node = null; private class Node { private String item; private Node next; } "inner class" public boolean isempty() {

11 Stack: Linked List Implementation public class LinkedStackOfStrings { private Node = null; private class Node { private String item; private Node next; } "inner class" public boolean isempty() { return == null; } public void push(string item) { Node second = ; = new Node();.item = item;.next = second; } stack and linked list contents after 4 th push operation } public String pop() { String item =.item; =.next; return item; } 18

12 Stack Data Structures: Tradeoffs Two data structures to implement Stack data type. Array. Every push/pop operation take constant time. But must fix maximum capacity of stack ahead of time. Linked list. Every push/pop operation takes constant time. Memory is proportional to number of items on stack. But uses extra space and time to deal with references. a[] to be or not N 20

13 List Processing Challenge 1 Q. What does the following code fragment do? for (Node x = ; x!= null; x = x.next) { StdOut.println(x.item); } Alice Bob Carol null item next 21

14 List Processing Challenge 2 Q. What does the following code fragment do? Node last = new Node(); last.item = StdIn.readString(); last.next = null; Node = last; while (!StdIn.isEmpty()) { last.next = new Node(); last = last.next; last.item = StdIn.readString(); last.next = null; } last Alice Bob Carol null item next 22

4.3 Stacks and Queues

4.3 Stacks and Queues Introduction to Programming in Java: An Interdisciplinary Approach Robert Sedgewick and Kevin Wayne Copyright 2002 2010 03/30/12 04:33:08 PM Data Types and Data Structures Data types.