StackAsLinkedList.java

public class StackAsLinkedList {
    // Time: push/pop/peek/isEmpty are all O(1).
    // Space: O(n) for n nodes.
  
    StackNode root; 
  
    static class StackNode { 
        int data; 
        StackNode next; 
  
        StackNode(int data) 
        { 
            this.data = data;
            this.next = null;
        } 
    } 
    
	
    public boolean isEmpty() 
    {
        return root == null;
    }
  
    public void push(int data) 
    {
        // Create a new node
        StackNode newNode = new StackNode(data);
        // New node points to current top
        newNode.next = root;
        // New node becomes the new top
        root = newNode;
    } 
  
    public int pop() 
    { 	

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