import java.util.ArrayList;

/*implementation of a stack using linked list. i.e stack size is dynamic*/

public class LinkedListStack {

public class Node{

private int key;

private String value;

private Node next;

public Node(int key, String value){

this.key = key;

this.value = value;

}

public String toString(){

return "[ (" + key + " , " + value + " )]";

}

}

private Node head;

private Node tail;

public void StackAsLinkedList(){

head = null;

tail = null;

}

public void push(int key, String value){

Node node = new Node(key, value);

if (head == null){

head = node;

tail = node;

} else {

Node temp = head;

head = node;

head.next = temp;

}

System.out.println(String.format("Push : value %s was added to stack", node));

}

public void pushMany(Node[] nodes){

for (Node node : nodes){

push(node.key, node.value);

}

}

public void pop(){

if ( head != null){

System.out.println(String.format("Pop : value %s was removed to stack", head));

head = head.next;

} else {

System.out.println("Sorry but Stack is already empty.");

}

}

public void popAll(){

while (head != null){

pop();

}

}

public void peek(){

if (head != null){

System.out.println(String.format("Peek : value %s is on top of stack", head));

} else {

System.out.println("Sorry but Stack is already empty.");

}

}

public String toString(){

ArrayList<String> list = new ArrayList<String>();

StringBuilder output = new StringBuilder();

System.out.println("Top of the Stack : " + head);

Node currentNode = head;

if (currentNode == null){

return output.toString();

}

while (currentNode != null){

list.add(currentNode.toString());

currentNode = currentNode.next;

}

for (String link : list){

output.append(" --> " + link);

}

return output.toString();

}

public static void main(String [] args){

LinkedListStack theStack = new LinkedListStack();

theStack.push(5, "Data Structures");

theStack.push(9, "Algorithms");

theStack.push(8, "Operating Systems");

theStack.push(7, "Data Mining");

System.out.println(theStack.toString());

theStack.peek();

theStack.pop();

System.out.println(theStack.toString());

}

}