Below is the syntax highlighted version of LinkedStack.java.
/****************************************************************************** * 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) * ******************************************************************************/ package edu.princeton.cs.algs4; 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)"); } } /****************************************************************************** * Copyright 2002-2022, Robert Sedgewick and Kevin Wayne. * * This file is part of algs4.jar, which accompanies the textbook * * Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne, * Addison-Wesley Professional, 2011, ISBN 0-321-57351-X. * http://algs4.cs.princeton.edu * * * algs4.jar is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * algs4.jar is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with algs4.jar. If not, see http://www.gnu.org/licenses. ******************************************************************************/