Java/src/main/java/com/thealgorithms/datastructures/stacks/NodeStack.java at master · TheAlgorithms/Java · GitHub

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packagecom.thealgorithms.datastructures.stacks;

/**
 * A stack implementation using linked nodes, supporting unlimited size without an ArrayList.
 *
 * <p>Each node in the stack contains data of generic type {@code Item}, along with references
 * to the next and previous nodes, supporting typical stack operations.
 *
 * <p>The stack follows a Last-In-First-Out (LIFO) order where elements added last are
 * removed first. Supported operations include push, pop, and peek.
 *
 * @param <Item> the type of elements held in this stack
 */
publicclassNodeStack<Item> {

/**
 * Node class representing each element in the stack.
 */
privateclassNode {
Itemdata;
Nodeprevious;

Node(Itemdata) {
this.data = data;
this.previous = null;
 }
 }

privateNodehead; // Top node in the stack
privateintsize; // Number of elements in the stack

/**
 * Constructs an empty NodeStack.
 */
publicNodeStack() {
head = null;
size = 0;
 }

/**
 * Pushes an item onto the stack.
 *
 * @param item the item to be pushed onto the stack
 */
publicvoidpush(Itemitem) {
NodenewNode = newNode(item);
newNode.previous = head;
head = newNode;
size++;
 }

/**
 * Removes and returns the item at the top of the stack.
 *
 * @return the item at the top of the stack, or {@code null} if the stack is empty
 * @throws IllegalStateException if the stack is empty
 */
publicItempop() {
if (isEmpty()) {
thrownewIllegalStateException("Cannot pop from an empty stack.");
 }
Itemdata = head.data;
head = head.previous;
size--;
returndata;
 }

/**
 * Returns the item at the top of the stack without removing it.
 *
 * @return the item at the top of the stack, or {@code null} if the stack is empty
 * @throws IllegalStateException if the stack is empty
 */
publicItempeek() {
if (isEmpty()) {
thrownewIllegalStateException("Cannot peek from an empty stack.");
 }
returnhead.data;
 }

/**
 * Checks whether the stack is empty.
 *
 * @return {@code true} if the stack has no elements, {@code false} otherwise
 */
publicbooleanisEmpty() {
returnhead == null;
 }

/**
 * Returns the number of elements currently in the stack.
 *
 * @return the size of the stack
 */
publicintsize() {
returnsize;
 }
}