package DataStructures.Stacks;

import java.util.Scanner;
import java.util.Stack;

/**
 * Reversal of a stack using recursion. 
 *
 * @author Ishika Agarwal, 2021
 */

public class ReverseStack {

    public static void main(String args[]){

        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the number of elements you wish to insert in the stack");
        int n = sc.nextInt();
        int i;
        Stack<Integer> stack = new Stack<Integer>();
        System.out.println("Enter the stack elements");
        for(i = 0; i < n ; i++)
            stack.push(sc.nextInt());
        sc.close();
        reverseStack(stack);
        System.out.println("The reversed stack is:");
        while(!stack.isEmpty()){
            System.out.print(stack.peek()+",");
            stack.pop();
        }

    }

    private static void reverseStack(Stack<Integer> stack) {
        if(stack.isEmpty()){
            return;
        }

        //Store the topmost element
        int element = stack.peek();
        //Remove the topmost element
        stack.pop();

        //Reverse the stack for the leftover elements
        reverseStack(stack);

        //Insert the topmost element to the bottom of the stack
        insertAtBottom(stack,element);

    }

    private static void insertAtBottom(Stack<Integer> stack, int element) {
        
        if(stack.isEmpty()){
            //When stack is empty, insert the element so it will be present at the bottom of the stack
            stack.push(element);
            return;
        }

        int ele = stack.peek();
        /*Keep popping elements till stack becomes empty. Push the elements once the topmost element has
            moved to the bottom of the stack.
        */
        stack.pop();
        insertAtBottom(stack, element);

        stack.push(ele);
    }
    
}