Data Structure and Algorithms — Stack

What is a Stack Data Structure?

A stack is a linear data structure that follows the Last In, First Out (LIFO) principle, which means the last element added to the stack will be the first one to be removed. Elements can only be inserted or deleted from one end of the stack, known as the top. This makes a stack an abstract data type with two primary operations: “push” (adding an element to the top) and “pop” (removing the top element).

Stack Representation:

A stack can be implemented using an array or a linked list. The array-based implementation is more common and straightforward. You use an array and a variable to keep track of the top element’s position.

Basic Operations on Stacks with their Complexity:

1. push(item): Insert an item onto the top of the stack : O(1) .

2. pop(): Remove and return the top element from the stack : O(1) .

3. peek(): Return the top element without removing it from the stack : O(1).

4. isEmpty(): Check if the stack is empty : O(1).

Here’s a simple implementation of a stack using an array in Java:

public class Stack {
    private int maxSize;
    private int[] stackArray;
    private int top;

    public Stack(int maxSize) {
        this.maxSize = maxSize;
        stackArray = new int[maxSize];
        top = -1;
    }

    public void push(int item) {
        if (top == maxSize - 1) {
            System.out.println("Stack is full. Cannot push " + item);
            return;
        }
        stackArray[++top] = item;
    }

    public int pop() {
        if (isEmpty()) {
            System.out.println("Stack is empty.");
            return -1;
        }
        return stackArray[top--];
    }

    public int peek() {
        if (isEmpty()) {
            System.out.println("Stack is empty.");
            return -1;
        }
        return stackArray[top];
    }

    public boolean isEmpty() {
        return (top == -1);
    }

    public int size() {
        return top + 1;
    }
}

Real-life Use Cases of Stack in Software Engineering:

Stacks have various real-life applications in software engineering and computer science, some of which include:

1. Function Call Stack: When a function is called, its variables and information are pushed onto the stack, and when the function returns, the information is popped off the stack.

2. Expression Evaluation: In compilers and interpreters, stacks are used to evaluate expressions and keep track of the operands and operators.

3. Undo/Redo Mechanisms: Many applications use stacks to implement undo and redo functionality. Each action is pushed onto a stack, allowing users to undo and redo operations.

4. Backtracking Algorithms: In algorithms like depth-first search, backtracking uses a stack to explore different paths in a search space.

5. Balanced Parentheses Checking: Stacks are used to check whether expressions with parentheses are balanced or not.

In conclusion, the stack data structure, with its last-in, first-out principle and efficient basic operations, serves as a fundamental tool in computer science, finding extensive use in diverse real-life applications, making it an essential concept for any software engineer to grasp.