# Computer Algorithms: Stack and Queue

## Introduction

Every developer knows that computer algorithms are tightly related to data structures. Indeed many of the algorithms depend on a data structures and can be very effective for some data structures and ineffective for others. A typical example of this is the heapsort algorithm, which depends on a data structure called “heap”. In this case although the stack and the queue are data structures instead of pure algorithms it’s imporant to understand their structure and the way they operate over data.

However, before we continue with the concrete realization of the stack and the queue, let’s first take a look on the definition of this term. A data structure is a logical abstraction that “models” the real world and presents (stores) our data in a specific format. The access to this data structure is often predefined thus we can access directly every item containing data. This help us to perform a different kind of tasks and operations over different kind of data structures – insert, delete, search, etc.. A typical data structures are the stack, the queue, the linked list and the tree.

All these structures help us perform specific operations effectively. For instance searching in a balanced tree is faster than searching in a linked list.

It is also very important to note that data structures can be represented in many different ways. We can model them using arrays or pointers, as shown in this post. In fact the most important thing is to represent the logical structure of the data structure you’re modeling. Thus the stack is a structure that follows the LIFO (Last In First Out) principle and it doesn’t matter how it is represented in our program (whether it will be coded with an array or with pointers). The important thing into a stack representation is to follow the LIFO principle correctly. In this case if the stack is an array only its top should be accessible and the only operation must be inserting new top of the stack.
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# Friday Algorithms: A Data Structure: JavaScript Stack

## Algorithms and Data Structures

Instead of writing about “pure” algorithms this time I’ve decided to write about data structures and to start with a stack implementation in JavaScript. However algorithms and data structures live together since the very beginning of the computer sciences. Another reason to write about data structures is that many algorithms need a specific data structure to be implemented. Most of the search algorithms are data structure dependent. You know that searching into a tree is different from searching into a linked list.

I’d like to write more about searching in my future algorithm posts, but first we need some data structure examples. The first one is, as I mentioned – stack.

Implemented in JavaScript this is really a simple example, which don’t need much to be understood. But in first place what is a stack?

You can thing of the “computer science” stack as a stack of sheets of paper, as it’s shown on the picture. You’ve three basic operations. You can add new element to the stack by putting it on the top of the stack, so the previous top of the stack becomes the second element in the stack. Another operation is to “pop” from the stack – remove the “first” element in the stack – the top most element, and to return it. And finally print the stack. This is difficult to define as an operation, but let say you’ve to show somehow every element from the stack.

Here’s a little diagram:

## Source Code

At the end some source code:

```var node = function() { var data; var next = null; }   var stack = function() { this.top = null;   this.push = function(data) { if (this.top == null) { this.top = new node(); this.top.data = data; } else { var temp = new node(); temp.data = data; temp.next = this.top; this.top = temp; } }   this.pop = function() { var temp = this.top; var data = this.top.data; this.top = this.top.next; temp = null; return data; }   this.print = function() { var node = this.top; while (node != null) { console.log(node.data); node = node.next; } } }   var s = new stack();   s.push(1); s.push(2); s.push(3);   s.print();   var a = s.pop();   s.print();```