/************************************************************************* * 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. * * Can be optimized by replacing full exchanges with half exchanges * (ala insertion sort). * *************************************************************************/ import java.util.Comparator; import java.util.Iterator; import java.util.NoSuchElementException; import edu.princeton.cs.introcs.StdIn; import edu.princeton.cs.introcs.StdOut; /** * The MinPQ class represents a priority queue of generic keys. * It supports the usual insert and delete-the-minimum * operations, along with methods for peeking at the minimum key, * testing if the priority queue is empty, and iterating through * the keys. * * This implementation uses a binary heap. * The insert and delete-the-minimum operations take * logarithmic amortized time. * The min , size , and is-empty operations take constant time. * Construction takes time proportional to the specified capacity or the number of * items used to initialize the data structure. * * For additional documentation, see Section 2.4 of * Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne */ public class MinPQ implements Iterable { private Key[] pq; // store items at indices 1 to N private int N; // number of items on priority queue private Comparator comparator; // optional comparator /** * Initializes an empty priority queue with the given initial capacity. * @param initCapacity the initial capacity of the priority queue */ 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. * @param initCapacity the initial capacity of the priority queue * @param comparator the order to use when comparing keys */ 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. * @param comparator the order to use when comparing keys */ public MinPQ(Comparator comparator) { this(1, comparator); } /** * Initializes a priority queue from the array of keys. * Takes time proportional to the number of keys, using sink-based heap construction. * @param keys the array of keys */ 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? * @return true if the priority queue is empty; false otherwise */ public boolean isEmpty() { return N == 0; } /** * Returns the number of keys on the priority queue. * @return the number of keys on the priority queue */ public int size() { return N; } /** * Returns a smallest key on the priority queue. * @return a smallest key on the priority queue * @throws java.util.NoSuchElementException if priority queue is empty */ public Key min() { if (isEmpty()) throw new NoSuchElementException("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; } /** * Adds a new key to the priority queue. * @param x the key to add 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(); } /** * Removes and returns a smallest key on the priority queue. * @return a smallest key on the priority queue * @throws java.util.NoSuchElementException if the priority queue is empty */ public Key delMin() { if (isEmpty()) throw new NoSuchElementException("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) 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 **********************************************************************/ /** * Returns an iterator that iterates over the keys on the priority queue * in ascending order. * The iterator doesn't implement remove() since it's optional. * @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(); } } /** * Unit tests the MinPQ data type. */ public static void main(String[] args) { MinPQ pq = new MinPQ(); 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)"); } }