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Stack

Stack: A Last-In-First-Out (LIFO) data structure where elements are added and removed from the top, similar to a stack of books. Common operations include:

Methods of Stack:

  • .empty(): boolean: Checks if the stack is empty or not.
  • .push(Object x): void: Adds an element to the top of the stack.
  • .pop(): Object: Removes and returns the topmost element.
  • .peek(): Object: Returns the topmost element without removing it.
  • .search(Object x): int: Returns the index of given element.

Stacks are commonly used for:

  • Undo/Redo functionality in text editors
  • Browser history navigation
  • Backtracking algorithms (e.g., solving mazes)
  • Managing function calls in programming (Call Stack)

Code Example:

Example Usage:

package DataStructures.stacks;

import java.util.Stack;

public class Main {
   
    public static void main(String[] args) {
        // Creating an instance of Stack (imported from java.util.Stack)
        Stack<String> stack = new Stack<>();

        // Checking if the stack is empty using .empty()
        System.out.println("Is stack empty? " + stack.empty());
        System.out.println(stack);

        // Adding items to the stack using .push(Object x)
        stack.push("Minecraft");
        stack.push("Detroit: Become Human");
        stack.push("League of Legends");
        stack.push("The Last of Us");
        stack.push("Elden Ring");

        // Checking if the stack is empty after adding items
        System.out.println("Is stack empty? " + stack.empty());

        // Printing the stack contents
        System.out.println(stack);
        System.out.println();

        // Removing the topmost item from the stack using .pop()
        stack.pop(); // removes "Elden Ring"
        String poppedGame = stack.pop(); // removes "The Last of Us" and assigns it to a variable
        System.out.println("Popped Game: " + poppedGame);
        System.out.println(stack);
        System.out.println();

        // Checking the topmost item in the stack using .peek()
        String peekedGame = stack.peek();
        System.out.println("Peeked Game: " + peekedGame);
        System.out.println(stack);
        System.out.println();

        // Searching for items in the stack using .search(Object x)
        int lol = stack.search("League of Legends");
        System.out.println("Index of League of Legends: " + lol); // returns 1, not 0
        int minecraft = stack.search("Minecraft");
        System.out.println("Index of Minecraft: " + minecraft); // returns 3
        int blasphemous = stack.search("Blasphemous");
        System.out.println("Index of Blasphemous: " + blasphemous); // returns -1

        /* Uncommenting the following code will cause an OutOfMemoryError
         * due to an infinite loop of pushing to the stack.
         *
         * while (true) {
         *     stack.push("Blasphemous");
         * }
         */
    }
}

Output:

Is stack empty? true
[]
Is stack empty? false
[Minecraft, Detroit: Become Human, League of Legends, The Last of Us, Elden Ring]

Popped Game: The Last of Us
[Minecraft, Detroit: Become Human, League of Legends]

Peeked Game: League of Legends
[Minecraft, Detroit: Become Human, League of Legends]

Index of League of Legends: 1
Index of Minecraft: 3
Index of Blasphemous: -1