Exercise_2.java

//Time Complexity : O(1)
//Space Complexity : O(n)
public class Exercise_2 {

 StackNode root; 
 
 static class StackNode {
 int data; 
 StackNode next; 
 
 StackNode(int data) 
 { 
 this.data = data;
 } 
 } 
 
	
 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);
 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;
 } else {
 int popped = root.data;
 root = root.next;
 return popped;
 }
 } 
 
 public int peek() 
 {
 if (isEmpty()) {
 System.out.println("Stack Underflow");
 return 0;
 } else {
 return root.data;
 }
 } 
 
	//Driver code
 public static void main(String[] args) 
 { 
 
 Exercise_2 sll = new Exercise_2();
 
 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()); 
 } 
}