What Is Stack in Data Structure and Algorithm?

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Heather Bennett

In data structure and algorithm, a stack is a linear data structure that follows the principle of Last In First Out (LIFO). It is designed to store and retrieve elements in a specific order, similar to how a stack of books or plates works.

How Does a Stack Work?

A stack operates on two main operations:

  • Push: This operation adds an element to the top of the stack.
  • Pop: This operation removes the topmost element from the stack.

The push operation increases the size of the stack by one, while the pop operation decreases it by one. The last element added to the stack is always the first one to be removed, hence the LIFO principle.

The Anatomy of a Stack

A stack can be visualized as a vertical arrangement of elements. It typically consists of two primary components:

  • Top: The top represents the most recently added element in the stack.
  • Base: The base represents the bottommost element in the stack.

All elements between the top and base are intermediate elements. As new elements are added, they push existing elements down, and when an element is removed, all elements above it move up to fill its place.

Example: Stack of Plates

To better understand how stacks work in real life, let’s consider an example of a stack of plates. When you wash dishes and start stacking them up, you place each new plate on top of the previous one. When you need to use or remove a plate later on, you always take the one on top, as it was the last one to be added.

Common Operations on a Stack

In addition to push and pop, stacks support a few other essential operations:

  • Peek: This operation allows you to view the topmost element without removing it from the stack.
  • IsEmpty: This operation checks whether the stack is empty or not. It returns true if there are no elements in the stack, and false otherwise.
  • Size: This operation returns the number of elements present in the stack at any given time.

Implementing a Stack

A stack can be implemented using various data structures, such as arrays or linked lists. In most programming languages, stacks are readily available as built-in data structures with pre-defined methods for performing push, pop, peek, and other operations.

An Array-Based Implementation

The simplest way to implement a stack is by using an array. The top of the stack can be represented by an index that points to the last element in the array. When pushing an element, this index increases by one; when popping an element, it decreases by one.

<code>
class Stack {
    constructor() {
        this.stack = [];
        this.top = -1;
    }
    
    push(element) {
        this.top++;
        this.stack[this.top] = element;
    }
    
    pop() {
        if (this.isEmpty()) {
            return "Stack Underflow";
        }
        
        const poppedElement = this.top];
        this.top--;
        
        return poppedElement;
    }
    
    peek() {
        if (this.isEmpty()) {
            return "Stack is Empty";
        }
        
        return this.top];
    }
    
    isEmpty() {
        return this.top === -1;
    }
    
    size() {
        return this.top + 1;
    }
}
</code>

With the above implementation, you can create a stack object and perform various operations such as push, pop, peek, isEmpty, and size on it.

Applications of Stacks

Stacks find wide applications in computer science and real-world scenarios. Some common examples include:

  • Function Call Stack: Stacks are used to keep track of function calls in programming languages.
  • Undo/Redo Operations: Stacks enable the undo/redo functionality in text editors and other software applications.
  • Backtracking Algorithms: Stacks are crucial for backtracking algorithms that involve exploring all possible solutions to a problem.
  • Balanced Parentheses: Stacks help validate whether parentheses in an expression are balanced or not.

In Conclusion

A stack is an important data structure in computer science that follows the LIFO principle. It supports fundamental operations like push, pop, peek, isEmpty, and size. Understanding stacks is essential for solving problems efficiently and implementing various algorithms.

Now that you have a good understanding of stacks, you can apply this knowledge to solve programming problems and optimize your code!

If you found this article helpful, please leave a comment below!

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