edu.princeton.cs.algs4.LinkedStack Maven / Gradle / Ivy
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/******************************************************************************
* 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 push and pop operations, along with methods
* for peeking at the top item, testing if the stack is empty, and iterating through
* the items in LIFO order.
*
* 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 push, pop, peek, size, and is-empty
* operations all take constant time in the worst case.
*
* For additional documentation,
* see Section 1.3 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class LinkedStack- implements Iterable
- {
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
- iterator() {
return new ListIterator();
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator
- {
private Node current = first;
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
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
stack = new LinkedStack();
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 2002-2018, 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.
******************************************************************************/