// License: GPL v3 or later courtesy of author Kevin Wayne
package edu.princeton.cs.algs4;
/*************************************************************************
* Compilation: javac Stack.java
* Execution: java Stack < input.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 Stack < tobe.txt
* to be not that or be (2 left on stack)
*
*************************************************************************/
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The <tt>Stack</tt> 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>
* All stack operations except iteration are constant time.
* <p>
* For additional documentation, see <a href="/algs4/13stacks">Section 1.3</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*/
public class Stack<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;
}
/**
* Create an empty stack.
*/
public Stack() {
first = null;
N = 0;
assert check();
}
/**
* Is the stack empty?
*/
public boolean isEmpty() {
return first == null;
}
/**
* Return the number of items in the stack.
*/
public int size() {
return N;
}
/**
* Add the item to the stack.
*/
public void push(Item item) {
Node oldfirst = first;
first = new Node();
first.item = item;
first.next = oldfirst;
N++;
assert check();
}
/**
* Delete and return the item most recently added to the stack.
* Throw an exception if no such item exists because the stack is empty.
*/
public Item pop() {
if (isEmpty()) throw new RuntimeException("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
}
/**
* Return the item most recently added to the stack.
* Throw an exception if no such item exists because the stack is empty.
*/
public Item peek() {
if (isEmpty()) throw new RuntimeException("Stack underflow");
return first.item;
}
/**
* Return string representation.
*/
@Override
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
// check internal invariants
private boolean check() {
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.next == null) return false;
}
// check internal consistency of instance variable N
int numberOfNodes = 0;
for (Node x = first; x != null; x = x.next) {
numberOfNodes++;
}
if (numberOfNodes != N) return false;
return true;
}
/**
* Return an iterator to the stack that iterates through the items in LIFO order.
*/
@Override
public Iterator<Item> iterator() { return new ListIterator(); }
// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator<Item> {
private Node current = first;
@Override
public boolean hasNext() { return current != null; }
@Override
public void remove() { throw new UnsupportedOperationException(); }
@Override
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* A test client.
*/
// public static void main(String[] args) {
// Stack<String> s = new Stack<String>();
// while (!StdIn.isEmpty()) {
// String item = StdIn.readString();
// if (!item.equals("-")) s.push(item);
// else if (!s.isEmpty()) StdOut.print(s.pop() + " ");
// }
// StdOut.println("(" + s.size() + " left on stack)");
// }
}