stack.ppt

Stack: Linked List
Implementation
 Push and pop at the head of the list
 New nodes should be inserted at the front of the list, so that they
become the top of the stack
 Nodes are removed from the front (top) of the list
 Straight-forward linked list implementation
 push and pop can be implemented fairly easily, e.g. assuming that
head is a reference to the node at the front of the list
public void push(int x){
// Make a new node whose next reference is
// the existing list
Node newNode = new Node(x, top);
top = newNode; // top points to new node
}

List Stack Example
Java Code
Stack st = new Stack();
st.push(6);
top
6

List Stack Example
Java Code
st.push(6);
st.push(1);
top
6
1

List Stack Example
Java Code
st.push(6);
st.push(1);
st.push(7);
top
6
1
7

List Stack Example
Java Code
st.push(6);
st.push(1);
st.push(7);
st.push(8);
top
6
1
7
8

List Stack Example
Java Code
st.push(6);
st.push(1);
st.push(7);
st.push(8);
st.pop();
top
6
1
7
8

List Stack Example
Java Code
st.push(6);
st.push(1);
st.push(7);
st.push(8);
st.pop();
top
6
1
7

Stack: ADT List
Implementation
 Push() and pop() either at the beginning or at the end
of ADT List
 at the beginning:
public void push(Object newItem) {
list.add(1, newItem);
} // end push
public Object pop() {
Object temp = list.get(1);
list.remove(1);
return temp;
} // end pop

Stack: ADT List
Implementation
of ADT List
 at the end:
public void push(Object newItem) {
list.add(list.size()+1, newItem);
} // end push
public Object pop() {
Object temp = list.get(list.size());
list.remove(list.size());
return temp;
} // end pop

Stack: ADT List
Implementation
of ADT List
 Efficiency depends on implementation of ADT List (not
guaranteed)
 On other hand: it was very fast to implement (code is
easy, unlikely that errors were introduced when
coding).

Applications of Stacks
 Call stack (recursion).
 Searching networks, traversing trees
(keeping a track where we are).
Examples:
 Checking balanced expressions
 Recognizing palindromes
 Evaluating algebraic expressions

Simple Applications of the ADT Stack:
Checking for Balanced Braces
 A stack can be used to verify whether a program
contains balanced braces
 An example of balanced braces
abc{defg{ijk}{l{mn}}op}qr
 An example of unbalanced braces
abc{def}}{ghij{kl}m
abc{def}{ghij{kl}m

 Requirements for balanced braces
 Each time you encounter a “}”, it matches an already
encountered “{”
 When you reach the end of the string, you have matched
each “{”

Figure 7-3
Traces of the algorithm that checks for balanced braces

Evaluating Postfix
Expressions
 A postfix (reverse Polish logic) calculator
 Requires you to enter postfix expressions
 Example: 2 3 4 + *
 When an operand is entered, the calculator
 Pushes it onto a stack
 When an operator is entered, the calculator
 Applies it to the top two operands of the stack
 Pops the operands from the stack
 Pushes the result of the operation on the stack

Evaluating Postfix
Expressions
Figure 7-8
The action of a postfix calculator when evaluating the expression 2 * (3 + 4)

Evaluating Postfix Expressions
 Pseudo code:
int evaluate(String expression)
{
Stack stack=new Stack(); // creaty empty stack
while (true) {
String c=expression.getNextItem();
if (c==ENDOFLINE)
return stack.pop();
if (c is operand)
stack.push(c);
else { // operation
int operand2=stack.pop();
int operand1=stack.pop();
stack.push(execute(c,operand1,operand2));
}
}
}

Queues
 A queue is a data structure that only allows items to
be inserted at the end and removed from the front
 “Queue” is the British word for a line (or line-up)
 Queues are FIFO (First In First Out) data structures
– “fair” data structures

What Can You Use a Queue
For?
 Processing inputs and outputs to screen (console)
 Server requests
 Instant messaging servers queue up incoming
messages
 Database requests
 Print queues
 One printer for dozens of computers
 Operating systems use queues to schedule CPU
jobs
 Simulations

Queue Operations
 A queue should implement (at least) these
operations:
 enqueue – insert an item at the back of the queue
 dequeue – remove an item from the front
 peek – return the item at the front of the queue without
removing it
 Like stacks it is assumed that these operations will
be implemented efficiently
 That is, in constant time

Queue: Array Implementation
 First consider using an array as the underlying
structure for a queue, one plan would be to
 Make the back of the queue the current size of the
queue (i.e., the number of elements stored)
 Make the front of the queue index 0
 Inserting an item can be performed in constant time
 But removing an item would require shifting all elements
in the queue to the left which is too slow!
 Therefore we need to find another way

An Array-Based
Implementation
Figure 8-8
a) A naive array-based implementation of a queue; b) rightward drift can cause the
queue to appear full

stack.ppt

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