LinkedStack.java

Below is the syntax highlighted version of LinkedStack.java from §1.3 Stacks and Queues.

/******************************************************************************
 *  Compilation:  javac LinkedStack.java
 *  Execution:    java LinkedStack < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/13stacks/tobe.txt
 *
 *  A generic stack, implemented using a linked list. Each stack
 *  element is of type Item.
 *
 *  % more tobe.txt
 *  to be or not to - be - - that - - - is
 *
 *  % java LinkedStack < tobe.txt
 *  to be not that or be (2 left on stack)
 *
 ******************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;


/**
 *  The {@code LinkedStack} class represents a last-in-first-out (LIFO) stack of
 *  generic items.
 *  It supports the usual <em>push</em> and <em>pop</em> operations, along with methods
 *  for peeking at the top item, testing if the stack is empty, and iterating through
 *  the items in LIFO order.
 *  <p>
 *  This implementation uses a singly linked list with a non-static nested class for
 *  linked-list nodes. See {@link Stack} for a version that uses a static nested class.
 *  The <em>push</em>, <em>pop</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em>
 *  operations all take constant time in the worst case.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class LinkedStack<Item> implements Iterable<Item> {
    private int n;          // size of the stack
    private Node first;     // top of stack

    // helper linked list class
    private class Node {
        private Item item;
        private Node next;
    }

    /**
     * Initializes an empty stack.
     */
    public LinkedStack() {
        first = null;
        n = 0;
        assert check();
    }

    /**
     * Is this stack empty?
     * @return true if this stack is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in the stack.
     * @return the number of items in the stack
     */
    public int size() {
        return n;
    }

    /**
     * Adds the item to this stack.
     * @param item the item to add
     */
    public void push(Item item) {
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        n++;
        assert check();
    }

    /**
     * Removes and returns the item most recently added to this stack.
     * @return the item most recently added
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item pop() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        Item item = first.item;        // save item to return
        first = first.next;            // delete first node
        n--;
        assert check();
        return item;                   // return the saved item
    }


    /**
     * Returns (but does not remove) the item most recently added to this stack.
     * @return the item most recently added to this stack
     * @throws java.util.NoSuchElementException if this stack is empty
     */
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Stack underflow");
        return first.item;
    }

    /**
     * Returns a string representation of this stack.
     * @return the sequence of items in the stack in LIFO order, separated by spaces
     */
    public String toString() {
        StringBuilder s = new StringBuilder();
        for (Item item : this)
            s.append(item + " ");
        return s.toString();
    }

    /**
     * Returns an iterator to this stack that iterates through the items in LIFO order.
     * @return an iterator to this stack that iterates through the items in LIFO order.
     */
    public Iterator<Item> iterator() {
        return new LinkedIterator();
    }

    // a linked-list iterator
    private class LinkedIterator implements Iterator<Item> {
        private Node current = first;

        public boolean hasNext() {
            return current != null;
        }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next;
            return item;
        }
    }


    // check internal invariants
    private boolean check() {

        // check a few properties of instance variable 'first'
        if (n < 0) {
            return false;
        }
        if (n == 0) {
            if (first != null) return false;
        }
        else if (n == 1) {
            if (first == null)      return false;
            if (first.next != null) return false;
        }
        else {
            if (first == null)      return false;
            if (first.next == null) return false;
        }

        // check internal consistency of instance variable n
        int numberOfNodes = 0;
        for (Node x = first; x != null && numberOfNodes <= n; x = x.next) {
            numberOfNodes++;
        }
        if (numberOfNodes != n) return false;

        return true;
    }

    /**
     * Unit tests the {@code LinkedStack} data type.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        LinkedStack<String> stack = new LinkedStack<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-"))
                stack.push(item);
            else if (!stack.isEmpty())
                StdOut.print(stack.pop() + " ");
        }
        StdOut.println("(" + stack.size() + " left on stack)");
    }
}