CS2006 - Data Structures I

CS2006 - Data Structures I Chapter 7StacksII

Topics • Implementation of ADT Stack • Using Linked lists • Using ADT List

Linked List Stack Implementation • Stacks can grow and shrink dynamically • No particular linked list variations are required: • The head pointer can also serve as the top of the stack. • Pushing becomes inserting the first node in a linked list. • Popping becomes removing the first node.

Linked List Stack Implementation • The head of List is the top of the stack head 1 3 7 9 1 top 3 7 9

LL Stack Implementation // Assumes that the classes StackInterface and Node are available public class StackRefereenceBased implements StackInterface { private Node top; // class Node is the same used in the LinkedList class except it stores // Objects rather than Comparables public StackRefereenceBased (){ top = null; } // end default constructor public boolean isEmpty() { return (top == null); } // end isEmpty

LL Stack Implementation (2) public void push(Object newItem) { top = new Node(newItem, top); } // end push public Object pop() throws StackException { if (!isEmpty()) { Object temp = top.getItem(); top = top.getNext(); return temp; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop

LL Stack Implementation (3) public void popAll() { top=null; } // end popAll public Object peek() throws StackException { if (!isEmpty()) { return top.getItem(); } else { throw new StackException("StackException on " + "peek: stack empty"); } // end else } // end peek } // end StackReferencedBased

ADT Stack ADT List Implementation • All stack operations can be implemented using the ADT List's operations • The implementation is simpler to write than the array-based and makes used of previously-written code (code reuse) • Position 1 of the list represents the top position of the stack

ADT Stack ADT List Implementation top List position 10 80 60 . . 5 1 2 3 List site()

ADT List Stack Implementation // Assumes that the classes ListInterface and Node are available public class StackListBased implements StackInterface { private ListInterface list; public StackListBased() { list = new ListReferenceBased(); } // end default constructor public boolean isEmpty() { return list.isEmpty (); } // end isEmpty

List Stack Implementation (2) public void push(Object newItem) { list.add (1, newItem); } // end push public Object pop() throws StackException { if (!list.isEmpty()) { Object temp = list.get(1); list.remove(1); return temp; } else { throw new StackException("StackException on " + "pop: stack empty"); } // end if } // end pop

List Stack Implementation (3) public void popAll() { list.removeAll(); } // end popAll public Object peek() throws StackException { if (!list.isEmpty()) { return list.get(1); } else { throw new StackException("StackException on " + "peek: stack empty"); } // end else } // end peek } // end StackListBased

ADT Stack Implementation Comparison • Array-Based • No overhead of reference manipulation • Prevents adding elements if the array is full • LL-Based • More complicated than ADT List • Most flexible, No size restrictions • ADT List-Based • Simpler to write • Not as efficient as using a linked list directly

Review • The ______ method of the ADT stack retrieves and then removes the top of the stack. • createStack • push • pop • peek

Review • The ______ method of the ADT stack retrieves the top of the stack, but does not change the stack. • createStack • push • pop • peek

Review • Which of the following methods of the ADT stack accepts a parameter? • push • pop • createStack • peek

Review • If a stack is used by an algorithm to check for balanced braces, which of the following is true once the end of the string is reached? • the stack is empty • the stack has one “{” • the stack has one “}” • the stack has one “{” and one “}”

Review • Which of the following operations of the ADT stack does not throw a StackException? • push • pop • popAll • peek

Review • The pop operation throws a StackException when it tries to ______. • add an item to an empty stack • add an item to an array-based implementation of a stack that is already full • delete an item from an array-based implementation of a stack that is already full • delete an item from an empty stack

Review • The push operation throws a StackException when it tries to ______. • add an item to an empty stack • add an item to an array-based implementation of a stack that is already full • delete an item from an array-based implementation of a stack that is already full • delete an item from an empty stack

Review • In the StackInterface class, the pop method returns an item that is an instance of ______. • Integer • Double • String • Object

CS2006 - Data Structures I