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;
    }

   /**
     * 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++;
    }

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

    // an iterator, doesn't implement remove() since it's optional
    private class LIFOIterator 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) {

       /***********************************************
        *  A stack of strings
        ***********************************************/
        Stack<String> s1 = new Stack<String>();
        s1.push("Vertigo");
        s1.push("Just Lose It");
        s1.push("Pieces of Me");
        System.out.println(s1.pop());
        s1.push("Drop It Like It's Hot");
        while (!s1.isEmpty()) {
            System.out.println(s1.pop());
        }
        System.out.println();

       /*********************************************************
        *  A stack of integers. Illustrates autoboxing and
        *  auto-unboxing.
        *********************************************************/
        Stack<Integer> s2 = new Stack<Integer>();
        for (int i = 0; i < 10; i++) {
            s2.push(i);
        }

        // test out the iterator
        for (int i : s2) {
            System.out.print(i + " ");
        }
        System.out.println();

        while (s2.size() >= 2) {
            int a = s2.pop();
            int b = s2.pop();
            int c = a + b;
            System.out.println(c);
            s2.push(a + b);
        }

    }
}


/*************************************************************************
 *  Compilation:  javac Stack.java
 *  Execution:    java Stack
 *
 *  A generic stack, implemented using a linked list. Each stack
 *  element is of type Item.
 *
 *  % java Stack
 *  Pieces of Me
 *  Drop It Like It's Hot
 *  Just Lose It
 *  Vertigo
 *
 *  17
 *  24
 *  30
 *  35
 *  39
 *  42
 *  44
 *  45
 *  45
 *
 *************************************************************************/

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Tagged as: Generic stack, Generic stack in java, Java program - Generic stack