MinPQ

import java.util.Comparator;
import java.util.Iterator;
import java.util.NoSuchElementException;
public class MinPQ implements Iterable{
    private Key[] pq;
    private int n;
    private Comparator comparator;
    /*
        Initializes an empty priority queue with the given initial capacity
    */
    public MinPQ(int initCapacity){
        pq = (Key[]) new Object[initCapacity + 1];
        n = 0;
    }
    /*
        Initializes an empty priority queue
    */
    public MinPQ(){
        this(1);
    }
    /*
        Initializes an empty priority queue with the given initial capacity 
        using the given comparator
    */
    public MinPQ(int initCapacity, Comparator comparator){
        this.comparator = comparator;
        pq = (Key[]) new Object[initCapacity + 1];
        n= 0;
    }
    /*
        Initializes an empty priority queue using the given comparator
    */
    public MinPQ(Comparator comparator){
        this(1,comparator);
    }
    /*
        Initializes a priority queue from the array of keys
    */
    public MinPQ(Key[] keys){
        n = keys.length;
        pq = (Key[]) new Object[keys.length+1];
        for (int i=0;i1] = keys[i];
        for (int k=n/2;k>=1;k--)
            sink(k);
        assert isMinHeap();
    }

    /*
        Returns true if this priority queue is empty
    */
    public boolean isEmpty(){
        return n==0;
    }
    /*
        Returns the number of keys on this priority queue
    */
    public int size(){
        return n;
    }
    /*
        Returns a smallest key on this priority queue
    */
    public Key min(){
        if(isEmpty()) throw new NoSuchElementException("Priority queue underflow");
        return pq[1];
    }
    /*
        helper function to double the size of the heap array
    */
    public 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;
    }
    /*
        Adds a new key to this 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();
    }
    /*
        Removes and returns a smallest key on this priority queue 
    */
    public Key delMin(){
        if (isEmpty()) throw new NoSuchElementException("Priority queue underflow");
        exch(1, n);
        Key min = pq[n--];
        sink(1);
        // n = n-1 n+1 = n
        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) 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;
        int 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);
    }
     /**
     * Returns an iterator that iterates over the keys on this priority queue
     * in ascending order. 
     *
     * @return an iterator that iterates over the keys in ascending order
     */
    public Iterator iterator() { return new HeapIterator(); }

    private class HeapIterator implements Iterator {
        // create a new pq
        private MinPQ 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(size());
            else                    copy = new MinPQ(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();
        }
    }
}