Stacks and Queues

Stacks and Queues 
 Stack
A stack is a container of objects that are inserted and removed according to the last-in first-out (LIFO) principle. In the push down stacks only two operations are allowed: push the item into the stack, and pop the item out of the stack.


 A stack is a limited access data structure - elements can be added and removed from the stack only at the top. push adds an item to the top of the stack, pop removes the item from the top. 


Applications
• The simplest application of a stack is to reverse a word. You push a given word to stack - letter by letter - and then pop letters from the stack.
• Another application is an "undo" mechanism in text editors; this operation is accomplished by keeping all text changes in a stack.


Queues
A queue is a container of objects (a linear collection) that are inserted and removed according to the first-in first-out (FIFO) principle.


 An excellent example of a queue is a line of students in the film ticket counter. New additions to a line made to the back of the queue, while removal (or serving) happens in the front. 
In the queue only two operations are allowed enqueue and dequeue. Enqueue means to insert an item into the back of the queue, dequeue means removing the front item. The picture demonstrates the FIFO access.


The difference between stacks and queues is in removing. In a stack we remove the item the most recently added; in a queue, we remove the item the least recently added.