content: /*************************************************************************
   *  Compilation:  javac MinPQ.java
   *  Execution:    java MinPQ < input.txt
   *  
   *  Generic min priority queue implementation with a binary heap.
   *  Can be used with a comparator instead of the natural order.
   *
   *  % java MinPQ < tinyPQ.txt
   *  E A E (6 left on pq)
   *
   *  We use a one-based array to simplify parent and child calculations.
   *
   *************************************************************************/
  
  import java.util.Comparator;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  /**
   *  The <tt>MinPQ</tt> class represents a priority queue of generic keys.
   *  It supports the usual <em>insert</em> and <em>delete-the-minimum</em>
   *  operations, along with methods for peeking at the maximum key,
   *  testing if the priority queue is empty, and iterating through
   *  the keys.
   *  <p>
   *  The <em>insert</em> and <em>delete-the-minimum</em> operations take
   *  logarithmic amortized time.
   *  The <em>min</em>, <em>size</em>, and <em>is-empty</em> operations take constant time.
   *  Construction takes time proportional to the specified capacity or the number of
   *  items used to initialize the data structure.
   *  <p>
   *  This implementation uses a binary heap.
   *  <p>
   *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/24pq">Section 2.4</a> of
   *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
   */
  public class MinPQ<Key> implements Iterable<Key> {
      private Key[] pq;                    // store items at indices 1 to N
      private int N;                       // number of items on priority queue
      private Comparator<Key> comparator;  // optional comparator
  
     /**
       * Create an empty priority queue with the given initial capacity.
       */
      public MinPQ(int initCapacity) {
          pq = (Key[]) new Object[initCapacity + 1];
          N = 0;
      }
  
     /**
       * Create an empty priority queue.
       */
      public MinPQ() { this(1); }
  
     /**
       * Create an empty priority queue with the given initial capacity,
       * using the given comparator.
       */
      public MinPQ(int initCapacity, Comparator<Key> comparator) {
          this.comparator = comparator;
          pq = (Key[]) new Object[initCapacity + 1];
          N = 0;
      }
  
     /**
       * Create an empty priority queue using the given comparator.
       */
      public MinPQ(Comparator<Key> comparator) { this(1, comparator); }
  
     /**
       * Create a priority queue with the given items.
       * Takes time proportional to the number of items using sink-based heap construction.
       */
      public MinPQ(Key[] keys) {
          N = keys.length;
          pq = (Key[]) new Object[keys.length + 1];
          for (int i = 0; i < N; i++)
              pq[i+1] = keys[i];
          for (int k = N/2; k >= 1; k--)
              sink(k);
          assert isMinHeap();
      }
  
     /**
       * Is the priority queue empty?
       */
      public boolean isEmpty() {
          return N == 0;
      }
  
     /**
       * Return the number of items on the priority queue.
       */
      public int size() {
          return N;
      }
  
     /**
       * Return the smallest key on the priority queue.
       * Throw an exception if no such key exists because the priority queue is empty.
       */
      public Key min() {
          if (isEmpty()) throw new RuntimeException("Priority queue underflow");
          return pq[1];
      }
  
      // helper function to double the size of the heap array
      private void resize(int capacity) {
          assert capacity > N;
          Key[] temp = (Key[]) new Object[capacity];
          for (int i = 1; i <= N; i++) temp[i] = pq[i];
          pq = temp;
      }
  
     /**
       * Add a new key to the priority queue.
       */
      public void insert(Key x) {
          // double size of array if necessary
          if (N == pq.length - 1) resize(2 * pq.length);
  
          // add x, and percolate it up to maintain heap invariant
          pq[++N] = x;
          swim(N);
          assert isMinHeap();
      }
  
     /**
       * Delete and return the smallest key on the priority queue.
       * Throw an exception if no such key exists because the priority queue is empty.
       */
      public Key delMin() {
          if (N == 0) throw new RuntimeException("Priority queue underflow");
          exch(1, N);
          Key min = pq[N--];
          sink(1);
          pq[N+1] = null;         // avoid loitering and help with garbage collection
          if ((N > 0) && (N == (pq.length - 1) / 4)) resize(pq.length  / 2);
          assert isMinHeap();
          return min;
      }
  
  
     /***********************************************************************
      * Helper functions to restore the heap invariant.
      **********************************************************************/
  
      private void swim(int k) {
          while (k > 1 && greater(k/2, k)) {
              exch(k, k/2);
              k = k/2;
          }
      }
  
      private void sink(int k) {
          while (2*k <= N) {
              int j = 2*k;
              if (j < N && greater(j, j+1)) j++;
              if (!greater(k, j)) break;
              exch(k, j);
              k = j;
          }
      }
  
     /***********************************************************************
      * Helper functions for compares and swaps.
      **********************************************************************/
      private boolean greater(int i, int j) {
          if (comparator == null) {
              return ((Comparable<Key>) pq[i]).compareTo(pq[j]) > 0;
          }
          else {
              return comparator.compare(pq[i], pq[j]) > 0;
          }
      }
  
      private void exch(int i, int j) {
          Key swap = pq[i];
          pq[i] = pq[j];
          pq[j] = swap;
      }
  
      // is pq[1..N] a min heap?
      private boolean isMinHeap() {
          return isMinHeap(1);
      }
  
      // is subtree of pq[1..N] rooted at k a min heap?
      private boolean isMinHeap(int k) {
          if (k > N) return true;
          int left = 2*k, right = 2*k + 1;
          if (left  <= N && greater(k, left))  return false;
          if (right <= N && greater(k, right)) return false;
          return isMinHeap(left) && isMinHeap(right);
      }
  
  
     /***********************************************************************
      * Iterators
      **********************************************************************/
  
     /**
       * Return an iterator that iterates over all of the keys on the priority queue
       * in ascending order.
       * <p>
       * The iterator doesn't implement <tt>remove()</tt> since it's optional.
       */
      public Iterator<Key> iterator() { return new HeapIterator(); }
  
      private class HeapIterator implements Iterator<Key> {
          // create a new pq
          private MinPQ<Key> copy;
  
          // add all items to copy of heap
          // takes linear time since already in heap order so no keys move
          public HeapIterator() {
              if (comparator == null) copy = new MinPQ<Key>(size());
              else                    copy = new MinPQ<Key>(size(), comparator);
              for (int i = 1; i <= N; i++)
                  copy.insert(pq[i]);
          }
  
          public boolean hasNext()  { return !copy.isEmpty();                     }
          public void remove()      { throw new UnsupportedOperationException();  }
  
          public Key next() {
              if (!hasNext()) throw new NoSuchElementException();
              return copy.delMin();
          }
      }
  
     /**
       * A test client.
       */
      public static void main(String[] args) {
          MinPQ<String> pq = new MinPQ<String>();
          while (!StdIn.isEmpty()) {
              String item = StdIn.readString();
              if (!item.equals("-")) pq.insert(item);
              else if (!pq.isEmpty()) StdOut.print(pq.delMin() + " ");
          }
          StdOut.println("(" + pq.size() + " left on pq)");
      }
  
  }
  