Java Program to Reverse a Stack Using Recursion

In the previous article, we have discussed about Java Program to Find HCF of Two Numbers Using Recursion

In this article we are going to see how we can reverse a stack using recursion by Java programming language.

Java Program to Reverse a Stack Using Recursion

Stack is a linear data structure which works on the principle of Last In First Out(LIFO). As per the problem statement we have to reverse all the elements of a stack.

Let’s see the program to understand it clearly.

Java Program to Reverse a Stack Using Recursion By Using Static Input Value
Java Program to Reverse a Stack Using Recursion By Using User Input Value

Method-1: Java Program to Reverse a Stack Using Recursion By Using Static Input Value

Approach:

Create an empty stack.
Push some elements onto the stack.
Print the original stack.
Call the user defined method rev( ) to reverse the stack. It runs a while loop and pushes the elements from top to the bottom using another method which uses recursion to contain the elements and push them.
Print the reversed stack.

Program:

import java.util.*;
// Main class
public class Main
{
    // Create a stack to operate on
    static Stack<Character> st = new Stack<>();
    // Recursive function to insert the element at bottom
    static void insertBottom(char x)
    {
        // Checks if stack is empty then pushes the element
        if(st.isEmpty())
            st.push(x);
        // If stack is not empty then holds all elements in the instances of recursive calls
        // until the end of the stack is reached
        else
        {
            char a = st.peek();
            st.pop();
            insertBottom(x);
            //once item is inserted at bottom push all other elements
            st.push(a);
        }
    }
    
    // Method to reverse the stack
    public static void rev()
    {
        if(st.size() > 0)
        {
            // contain all elements until we reach the end of the stack then push
            char x = st.peek();
            st.pop();
            rev();
            // push the item at bottom
            insertBottom(x);
        }
    }

    public static void main(String[] args)
    {
        // Static initialisation of elements to the stack
        st.push('1');
        st.push('2');
        st.push('5');
        st.push('6');
        st.push('9');
        st.push('0');
        // Print the original stack
        System.out.println("Original Stack");
        System.out.println(st);
        // call the reverse method to reverse the stack
        rev();
        // Print the reversed stack
        System.out.println("Reversed Stack");
        System.out.println(st);
    }
}

Output:

Original Stack
[1, 2, 5, 6, 9, 0]
Reversed Stack
[0, 9, 6, 5, 2, 1]

Method-2: Java Program to Reverse a Stack Using Recursion By Using User Input Value

Approach:

Create an empty stack.
Ask the user to enter the number of elements.
Use a for loops to store and push the elements to the stack.
Print the original stack.
Call the user defined method rev( ) to reverse the stack. It runs a while loop and pushes the elements from top to the bottom using another method which uses recursion to contain the elements and push them.
Print the reversed stack.

Program:

import java.util.*;
// Main class
public class Main
{
    // Create a stack to operate on
    static Stack<Character> st = new Stack<>();
    // Recursive function to insert the element at bottom
    static void insertBottom(char x)
    {
        // Checks if stack is empty then pushes the element
        if(st.isEmpty())
            st.push(x);
        // If stack is not empty then holds all elements in the instances of recursive calls
        // until the end of the stack is reached
        else
        {
            char a = st.peek();
            st.pop();
            insertBottom(x);
            //once item is inserted at bottom push all other elements
            st.push(a);
        }
    }
    
    // Method to reverse the stack
    public static void rev()
    {
        if(st.size() > 0)
        {
            // contain all elements until we reach the end of the stack then push
            char x = st.peek();
            st.pop();
            rev();
            // push the item at bottom
            insertBottom(x);
        }
    }

    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        // Asks the user for input
        System.out.println("How many elements you want to enter ?");
        int num = sc.nextInt();
        char ch;
        // Ask the user to enter the elements and push them onto the stack
        for(int i = 0; i < num; i++)
        {
            System.out.println("Enter "+(i+1)+"th element!");
            ch = sc.next().charAt(0);
            st.push(ch);
        }
        // Print the original stack
        System.out.println("Original Stack");
        System.out.println(st);
        // call the reverse method to reverse the stack
        rev();
        // Print the reversed stack
        System.out.println("Reversed Stack");
        System.out.println(st);
    }
}

Output:

How many elements you want to enter ?
3
Enter 1th element!
A
Enter 2th element!
B
Enter 3th element!
C
Original Stack
[A, B, C]
Reversed Stack
[C, B, A]

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