Below is the syntax highlighted version of IndexMaxPQ.java
from §2.4 Priority Queues.
/****************************************************************************** * Compilation: javac IndexMaxPQ.java * Execution: java IndexMaxPQ * Dependencies: StdOut.java * * Maximum-oriented indexed PQ implementation using a binary heap. * ******************************************************************************/ import java.util.Iterator; import java.util.NoSuchElementException; /** * The {@code IndexMaxPQ} class represents an indexed priority queue of generic keys. * It supports the usual <em>insert</em> and <em>delete-the-maximum</em> * operations, along with <em>delete</em> and <em>change-the-key</em> * methods. In order to let the client refer to items on the priority queue, * an integer between {@code 0} and {@code maxN - 1} * is associated with each key—the client * uses this integer to specify which key to delete or change. * It also supports methods for peeking at a maximum key, * testing if the priority queue is empty, and iterating through * the keys. * <p> * This implementation uses a <em>binary heap</em> along with an * array to associate keys with integers in the given range. * The <em>insert</em>, <em>delete-the-maximum</em>, <em>delete</em>, * <em>change-key</em>, <em>decrease-key</em>, and <em>increase-key</em> * operations take Θ(log <em>n</em>) time in the worst case, * where <em>n</em> is the number of elements in the priority queue. * Construction takes time proportional to the specified capacity. * <p> * For additional documentation, see * <a href="https://algs4.cs.princeton.edu/24pq">Section 2.4</a> of * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne * * @param <Key> the generic type of key on this priority queue */ public class IndexMaxPQ<Key extends Comparable<Key>> implements Iterable<Integer> { private int maxN; // maximum number of elements on PQ private int n; // number of elements on PQ private int[] pq; // binary heap using 1-based indexing private int[] qp; // inverse of pq - qp[pq[i]] = pq[qp[i]] = i private Key[] keys; // keys[i] = priority of i /** * Initializes an empty indexed priority queue with indices between {@code 0} * and {@code maxN - 1}. * * @param maxN the keys on this priority queue are index from {@code 0} to {@code maxN - 1} * @throws IllegalArgumentException if {@code maxN < 0} */ public IndexMaxPQ(int maxN) { if (maxN < 0) throw new IllegalArgumentException(); this.maxN = maxN; n = 0; keys = (Key[]) new Comparable[maxN + 1]; // make this of length maxN?? pq = new int[maxN + 1]; qp = new int[maxN + 1]; // make this of length maxN?? for (int i = 0; i <= maxN; i++) qp[i] = -1; } /** * Returns true if this priority queue is empty. * * @return {@code true} if this priority queue is empty; * {@code false} otherwise */ public boolean isEmpty() { return n == 0; } /** * Is {@code i} an index on this priority queue? * * @param i an index * @return {@code true} if {@code i} is an index on this priority queue; * {@code false} otherwise * @throws IllegalArgumentException unless {@code 0 <= i < maxN} */ public boolean contains(int i) { validateIndex(i); return qp[i] != -1; } /** * Returns the number of keys on this priority queue. * * @return the number of keys on this priority queue */ public int size() { return n; } /** * Associate key with index i. * * @param i an index * @param key the key to associate with index {@code i} * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @throws IllegalArgumentException if there already is an item * associated with index {@code i} */ public void insert(int i, Key key) { validateIndex(i); if (contains(i)) throw new IllegalArgumentException("index is already in the priority queue"); n++; qp[i] = n; pq[n] = i; keys[i] = key; swim(n); } /** * Returns an index associated with a maximum key. * * @return an index associated with a maximum key * @throws NoSuchElementException if this priority queue is empty */ public int maxIndex() { if (n == 0) throw new NoSuchElementException("Priority queue underflow"); return pq[1]; } /** * Returns a maximum key. * * @return a maximum key * @throws NoSuchElementException if this priority queue is empty */ public Key maxKey() { if (n == 0) throw new NoSuchElementException("Priority queue underflow"); return keys[pq[1]]; } /** * Removes a maximum key and returns its associated index. * * @return an index associated with a maximum key * @throws NoSuchElementException if this priority queue is empty */ public int delMax() { if (n == 0) throw new NoSuchElementException("Priority queue underflow"); int max = pq[1]; exch(1, n--); sink(1); assert pq[n+1] == max; qp[max] = -1; // delete keys[max] = null; // to help with garbage collection pq[n+1] = -1; // not needed return max; } /** * Returns the key associated with index {@code i}. * * @param i the index of the key to return * @return the key associated with index {@code i} * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @throws NoSuchElementException no key is associated with index {@code i} */ public Key keyOf(int i) { validateIndex(i); if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue"); else return keys[i]; } /** * Change the key associated with index {@code i} to the specified value. * * @param i the index of the key to change * @param key change the key associated with index {@code i} to this key * @throws IllegalArgumentException unless {@code 0 <= i < maxN} */ public void changeKey(int i, Key key) { validateIndex(i); if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue"); keys[i] = key; swim(qp[i]); sink(qp[i]); } /** * Change the key associated with index {@code i} to the specified value. * * @param i the index of the key to change * @param key change the key associated with index {@code i} to this key * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @deprecated Replaced by {@code changeKey(int, Key)}. */ @Deprecated public void change(int i, Key key) { validateIndex(i); changeKey(i, key); } /** * Increase the key associated with index {@code i} to the specified value. * * @param i the index of the key to increase * @param key increase the key associated with index {@code i} to this key * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @throws IllegalArgumentException if {@code key <= keyOf(i)} * @throws NoSuchElementException no key is associated with index {@code i} */ public void increaseKey(int i, Key key) { validateIndex(i); if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue"); if (keys[i].compareTo(key) == 0) throw new IllegalArgumentException("Calling increaseKey() with a key equal to the key in the priority queue"); if (keys[i].compareTo(key) > 0) throw new IllegalArgumentException("Calling increaseKey() with a key that is strictly less than the key in the priority queue"); keys[i] = key; swim(qp[i]); } /** * Decrease the key associated with index {@code i} to the specified value. * * @param i the index of the key to decrease * @param key decrease the key associated with index {@code i} to this key * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @throws IllegalArgumentException if {@code key >= keyOf(i)} * @throws NoSuchElementException no key is associated with index {@code i} */ public void decreaseKey(int i, Key key) { validateIndex(i); if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue"); if (keys[i].compareTo(key) == 0) throw new IllegalArgumentException("Calling decreaseKey() with a key equal to the key in the priority queue"); if (keys[i].compareTo(key) < 0) throw new IllegalArgumentException("Calling decreaseKey() with a key that is strictly greater than the key in the priority queue"); keys[i] = key; sink(qp[i]); } /** * Remove the key on the priority queue associated with index {@code i}. * * @param i the index of the key to remove * @throws IllegalArgumentException unless {@code 0 <= i < maxN} * @throws NoSuchElementException no key is associated with index {@code i} */ public void delete(int i) { validateIndex(i); if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue"); int index = qp[i]; exch(index, n--); swim(index); sink(index); keys[i] = null; qp[i] = -1; } // throw an IllegalArgumentException if i is an invalid index private void validateIndex(int i) { if (i < 0) throw new IllegalArgumentException("index is negative: " + i); if (i >= maxN) throw new IllegalArgumentException("index >= capacity: " + i); } /*************************************************************************** * General helper functions. ***************************************************************************/ private boolean less(int i, int j) { return keys[pq[i]].compareTo(keys[pq[j]]) < 0; } private void exch(int i, int j) { int swap = pq[i]; pq[i] = pq[j]; pq[j] = swap; qp[pq[i]] = i; qp[pq[j]] = j; } /*************************************************************************** * Heap helper functions. ***************************************************************************/ private void swim(int k) { while (k > 1 && less(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 && less(j, j+1)) j++; if (!less(k, j)) break; exch(k, j); k = j; } } /** * Returns an iterator that iterates over the keys on the * priority queue in descending order. * The iterator doesn't implement {@code remove()} since it's optional. * * @return an iterator that iterates over the keys in descending order */ public Iterator<Integer> iterator() { return new HeapIterator(); } private class HeapIterator implements Iterator<Integer> { // create a new pq private IndexMaxPQ<Key> copy; // add all elements to copy of heap // takes linear time since already in heap order so no keys move public HeapIterator() { copy = new IndexMaxPQ<Key>(pq.length - 1); for (int i = 1; i <= n; i++) copy.insert(pq[i], keys[pq[i]]); } public boolean hasNext() { return !copy.isEmpty(); } public void remove() { throw new UnsupportedOperationException(); } public Integer next() { if (!hasNext()) throw new NoSuchElementException(); return copy.delMax(); } } /** * Unit tests the {@code IndexMaxPQ} data type. * * @param args the command-line arguments */ public static void main(String[] args) { // insert a bunch of strings String[] strings = { "it", "was", "the", "best", "of", "times", "it", "was", "the", "worst" }; IndexMaxPQ<String> pq = new IndexMaxPQ<String>(strings.length); for (int i = 0; i < strings.length; i++) { pq.insert(i, strings[i]); } // print each key using the iterator for (int i : pq) { StdOut.println(i + " " + strings[i]); } StdOut.println(); // increase or decrease the key for (int i = 0; i < strings.length; i++) { if (StdRandom.bernoulli(0.5)) pq.increaseKey(i, strings[i] + strings[i]); else pq.decreaseKey(i, strings[i].substring(0, 1)); } // delete and print each key while (!pq.isEmpty()) { String key = pq.maxKey(); int i = pq.delMax(); StdOut.println(i + " " + key); } StdOut.println(); // reinsert the same strings for (int i = 0; i < strings.length; i++) { pq.insert(i, strings[i]); } // delete them in random order int[] perm = new int[strings.length]; for (int i = 0; i < strings.length; i++) perm[i] = i; StdRandom.shuffle(perm); for (int i = 0; i < perm.length; i++) { String key = pq.keyOf(perm[i]); pq.delete(perm[i]); StdOut.println(perm[i] + " " + key); } } }