Exercise_2.java

class StackAsLinkedList {
    // Time Complexity: o(1)
    // Space Complexity: o(N) where N being size of the data;
    StackNode root;

    static class StackNode {
        int data;
        StackNode next;

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


    public boolean isEmpty()
    {
        //Write your code here for the condition if stack is empty.
        return root == null;
    }

    public void push(int data)
    {
        //Write code to push data to the stack.
        StackNode newNode = new StackNode(data);
        if (isEmpty()) {
            root = newNode;
        } else {
            newNode.next = root;
            root = newNode;
        }
    }

    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
        if (isEmpty()) {
            System.out.println("Stack Underflow");
            return 0;
        }
        int poped = root.data;
        root = root.next;
        return poped;
    }

    public int peek()
    {
        //Write code to just return the topmost element without removing it.
        if (isEmpty()) {
            System.out.println("Stack is Empty");
            return -1;
        }
        return root.data;
    }

	//Driver code
    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("Top element is " + sll.peek());
    }
}