package com.felix.util;
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.
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
/**
* Return string representation.
*/
public String[] toStringArray() {
String[] s = new String[N];
int i = 0;
for (Item item : this)
s[i++] = (String) item;
return s;
}
/**
* Return string representation.
*/
public String[] toStringArrayReverse() {
String[] s = new String[N];
int i = N-1;
for (Item item : this)
s[i--] = (String) item;
return s;
}
// 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.
*/
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;
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;
}
}
/**
* 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)");
}
}