Exercise_2.java

class StackAsLinkedList {

    StackNode root;

    static class StackNode {
        int data;
        StackNode next;

        StackNode(int data) {
            //Constructor here
            this.data = data;
            this.next = null;
        }
    }


    public boolean isEmpty() {
        // Time Complexity : O(1)
        // Space Complexity : O(1)

        if (root == null) {
            return true;
        } else {
            return false;
        }
        //Write your code here for the condition if stack is empty.
    }

    public void push(int data) {
        // Time Complexity : O(1)
        // Space Complexity : O(1)

        //Write code to push data to the stack.
        if (root == null) {
            root = new StackNode(data);
            return;
        }
        StackNode node = new StackNode(data);
        node.next = root;
        root = node;

    }

    public int pop() {
        //If Stack Empty Return 0 and print "Stack Underflow"
        //Write code to pop the topmost element of stack.
        //Also return the popped element

        // Time Complexity : O(1)
        // Space Complexity : O(1)

        if (root == null) {
            System.out.print("***** Stack Underflow : ");
            return 0;
        }
        int popped = root.data;
        root = root.next;
        return popped;
    }

    public int peek() {
        //Write code to just return the topmost element without removing it.
        if (root == null) {
            return 0;
        }
        return root.data;
    }
}
class Main {
    public static void main(String args[])
    {

        StackAsLinkedList sll = new StackAsLinkedList();

        sll.push(10);
        sll.push(20);
        sll.push(30);

        System.out.println(sll.pop() + " popped from stack");
        System.out.println(sll.pop() + " popped from stack");
        System.out.println(sll.pop() + " popped from stack");
        System.out.println(sll.pop() + " popped from stack");


        System.out.println("Top element is " + sll.peek());
    }
}