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Enhance docs, remove main, add tests in NodeStack (#6017)
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‎DIRECTORY.md

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* [QueueByTwoStacksTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/QueueByTwoStacksTest.java)
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* [QueueTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/queues/QueueTest.java)
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* stacks
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* [NodeStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/stacks/NodeStackTest.java)
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* [StackArrayListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/stacks/StackArrayListTest.java)
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* [StackArrayTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/stacks/StackArrayTest.java)
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* [StackOfLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/stacks/StackOfLinkedListTest.java)

‎src/main/java/com/thealgorithms/datastructures/stacks/NodeStack.java

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package com.thealgorithms.datastructures.stacks;
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/**
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* Implementation of a stack using nodes. Unlimited size, no arraylist.
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* A stack implementation using linked nodes, supporting unlimited size without an ArrayList.
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*
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* @author Kyler Smith, 2017
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* <p>Each node in the stack contains data of generic type {@code Item}, along with references
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* to the next and previous nodes, supporting typical stack operations.
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*
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* <p>The stack follows a Last-In-First-Out (LIFO) order where elements added last are
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* removed first. Supported operations include push, pop, and peek.
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*
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* @param <Item> the type of elements held in this stack
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*/
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public class NodeStack<Item> {
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/**
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* Entry point for the program.
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* Node class representing each element in the stack.
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*/
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public static void main(String[] args) {
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NodeStack<Integer> stack = new NodeStack<Integer>();
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stack.push(3);
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stack.push(4);
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stack.push(5);
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System.out.println("Testing :");
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stack.print(); // prints : 5 4 3
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private class Node {
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Item data;
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Node previous;
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Integer x = stack.pop(); // x = 5
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stack.push(1);
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stack.push(8);
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Integer y = stack.peek(); // y = 8
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System.out.println("Testing :");
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stack.print(); // prints : 8 1 4 3
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System.out.println("Testing :");
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System.out.println("x : " + x);
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System.out.println("y : " + y);
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Node(Item data) {
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this.data = data;
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this.previous = null;
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}
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}
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/**
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* Information each node should contain.
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*
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* @value data : information of the value in the node
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* @value head : the head of the stack
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* @value next : the next value from this node
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* @value previous : the last value from this node
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* @value size : size of the stack
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*/
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private Item data;
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private static NodeStack<?> head;
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private NodeStack<?> previous;
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private static int size = 0;
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private Node head; // Top node in the stack
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private int size; // Number of elements in the stack
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/**
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* Constructors for the NodeStack.
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* Constructs an empty NodeStack.
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*/
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public NodeStack() {
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}
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private NodeStack(Item item) {
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this.data = item;
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head = null;
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size = 0;
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}
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/**
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* Put a value onto the stack.
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* Pushes an item onto the stack.
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*
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* @param item : value to be put on the stack.
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* @param item the item to be pushed onto the stack
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*/
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public void push(Item item) {
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NodeStack<Item> newNs = new NodeStack<Item>(item);
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if (this.isEmpty()) {
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NodeStack.setHead(new NodeStack<>(item));
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newNs.setNext(null);
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newNs.setPrevious(null);
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} else {
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newNs.setPrevious(NodeStack.head);
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NodeStack.head.setNext(newNs);
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NodeStack.setHead(newNs);
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}
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NodeStack.setSize(NodeStack.getSize() + 1);
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Node newNode = new Node(item);
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newNode.previous = head;
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head = newNode;
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size++;
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}
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/**
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* Value to be taken off the stack.
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* Removes and returns the item at the top of the stack.
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*
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* @return item : value that is returned.
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* @return the item at the top of the stack, or {@code null} if the stack is empty
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* @throws IllegalStateException if the stack is empty
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*/
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public Item pop() {
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Item item = (Item) NodeStack.head.getData();
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NodeStack.setHead(NodeStack.head.getPrevious());
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NodeStack.head.setNext(null);
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NodeStack.setSize(NodeStack.getSize() - 1);
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return item;
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if (isEmpty()) {
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throw new IllegalStateException("Cannot pop from an empty stack.");
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}
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Item data = head.data;
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head = head.previous;
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size--;
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return data;
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}
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/**
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* Value that is next to be taken off the stack.
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* Returns the item at the top of the stack without removing it.
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*
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* @return item : the next value that would be popped off the stack.
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* @return the item at the top of the stack, or {@code null} if the stack is empty
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* @throws IllegalStateException if the stack is empty
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*/
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public Item peek() {
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return (Item) NodeStack.head.getData();
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if (isEmpty()) {
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throw new IllegalStateException("Cannot peek from an empty stack.");
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}
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return head.data;
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}
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/**
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* If the stack is empty or there is a value in.
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* Checks whether the stack is empty.
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*
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* @return boolean : whether or not the stack has anything in it.
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* @return {@code true} if the stack has no elements, {@code false} otherwise
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*/
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public boolean isEmpty() {
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return NodeStack.getSize() == 0;
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return head == null;
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}
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/**
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* Returns the size of the stack.
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* Returns the number of elements currently in the stack.
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*
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* @return int : number of values in the stack.
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* @return the size of the stack
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*/
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public int size() {
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return NodeStack.getSize();
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return size;
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}
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/**
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* Print the contents of the stack in the following format.
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*
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* <p>
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* x <- head (next out) y z <- tail (first in) . . .
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* Prints the contents of the stack from top to bottom.
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*/
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public void print() {
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for (NodeStack<?> n = NodeStack.head; n != null; n = n.previous) {
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System.out.println(n.getData().toString());
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Node current = head;
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while (current != null) {
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System.out.println(current.data);
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current = current.previous;
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}
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}
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private static void setHead(NodeStack<?> ns) {
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NodeStack.head = ns;
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}
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private void setNext(NodeStack<?> next) {
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}
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private NodeStack<?> getPrevious() {
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return previous;
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}
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private void setPrevious(NodeStack<?> previous) {
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this.previous = previous;
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}
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private static int getSize() {
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return size;
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}
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private static void setSize(int size) {
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NodeStack.size = size;
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}
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private Item getData() {
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return this.data;
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}
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}

‎src/test/java/com/thealgorithms/datastructures/stacks/NodeStackTest.java

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package com.thealgorithms.datastructures.stacks;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import static org.junit.jupiter.api.Assertions.assertFalse;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import static org.junit.jupiter.api.Assertions.assertTrue;
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import org.junit.jupiter.api.Test;
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class NodeStackTest {
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@Test
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void testPush() {
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NodeStack<Integer> stack = new NodeStack<>();
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stack.push(10);
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stack.push(20);
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assertEquals(20, stack.peek(), "Top element should be 20 after pushing 10 and 20.");
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}
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@Test
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void testPop() {
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NodeStack<String> stack = new NodeStack<>();
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stack.push("First");
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stack.push("Second");
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assertEquals("Second", stack.pop(), "Pop should return 'Second', the last pushed element.");
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assertEquals("First", stack.pop(), "Pop should return 'First' after 'Second' is removed.");
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}
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@Test
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void testPopOnEmptyStack() {
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NodeStack<Double> stack = new NodeStack<>();
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assertThrows(IllegalStateException.class, stack::pop, "Popping an empty stack should throw IllegalStateException.");
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}
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@Test
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void testPeek() {
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NodeStack<Integer> stack = new NodeStack<>();
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stack.push(5);
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stack.push(15);
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assertEquals(15, stack.peek(), "Peek should return 15, the top element.");
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stack.pop();
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assertEquals(5, stack.peek(), "Peek should return 5 after 15 is popped.");
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}
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@Test
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void testPeekOnEmptyStack() {
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NodeStack<String> stack = new NodeStack<>();
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assertThrows(IllegalStateException.class, stack::peek, "Peeking an empty stack should throw IllegalStateException.");
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}
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@Test
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void testIsEmpty() {
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NodeStack<Character> stack = new NodeStack<>();
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assertTrue(stack.isEmpty(), "Newly initialized stack should be empty.");
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stack.push('A');
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assertFalse(stack.isEmpty(), "Stack should not be empty after a push operation.");
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stack.pop();
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assertTrue(stack.isEmpty(), "Stack should be empty after popping the only element.");
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}
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@Test
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void testSize() {
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NodeStack<Integer> stack = new NodeStack<>();
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assertEquals(0, stack.size(), "Size of empty stack should be 0.");
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stack.push(3);
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stack.push(6);
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assertEquals(2, stack.size(), "Size should be 2 after pushing two elements.");
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stack.pop();
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assertEquals(1, stack.size(), "Size should be 1 after popping one element.");
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stack.pop();
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assertEquals(0, stack.size(), "Size should be 0 after popping all elements.");
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}
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@Test
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void testPrint() {
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NodeStack<Integer> stack = new NodeStack<>();
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stack.push(1);
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stack.push(2);
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stack.push(3);
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// Output verification would ideally be handled through a different means
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// but you can print as a basic check to confirm method runs without errors.
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stack.print();
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}
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}

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