Heap.java

Below is the syntax highlighted version of Heap.java from §2.4 Priority Queues.

/*************************************************************************
 *  Compilation:  javac Heap.java
 *  Execution:    java Heap < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   http://algs4.cs.princeton.edu/24pq/tiny.txt
 *                http://algs4.cs.princeton.edu/24pq/words3.txt
 *  
 *  Sorts a sequence of strings from standard input using heapsort.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java Heap < tiny.txt
 *  S O R T E X A M P L E A               [ one string per line ]
 *
 *  % more words3.txt
 *  bed bug dad yes zoo ... all bad yet
 *
 *  % java Heap < words3.txt
 *  all bad bed bug dad ... yes yet zoo   [ one string per line ]
 *
 *************************************************************************/

public class Heap {

    public static void sort(Comparable[] pq) {
        int N = pq.length;
        for (int k = N/2; k >= 1; k--)
            sink(pq, k, N);
        while (N > 1) {
            exch(pq, 1, N--);
            sink(pq, 1, N);
        }
    }

   /***********************************************************************
    * Helper functions to restore the heap invariant.
    **********************************************************************/

    private static void sink(Comparable[] pq, int k, int N) {
        while (2*k <= N) {
            int j = 2*k;
            if (j < N && less(pq, j, j+1)) j++;
            if (!less(pq, k, j)) break;
            exch(pq, k, j);
            k = j;
        }
    }

   /***********************************************************************
    * Helper functions for comparisons and swaps.
    * Indices are "off-by-one" to support 1-based indexing.
    **********************************************************************/
    private static boolean less(Comparable[] pq, int i, int j) {
        return pq[i-1].compareTo(pq[j-1]) < 0;
    }

    private static void exch(Object[] pq, int i, int j) {
        Object swap = pq[i-1];
        pq[i-1] = pq[j-1];
        pq[j-1] = swap;
    }

    // is v < w ?
    private static boolean less(Comparable v, Comparable w) {
        return (v.compareTo(w) < 0);
    }
        

   /***********************************************************************
    *  Check if array is sorted - useful for debugging
    ***********************************************************************/
    private static boolean isSorted(Comparable[] a) {
        for (int i = 1; i < a.length; i++)
            if (less(a[i], a[i-1])) return false;
        return true;
    }


    // print array to standard output
    private static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++) {
            StdOut.println(a[i]);
        }
    }

    // Read strings from standard input, sort them, and print.
    public static void main(String[] args) {
        String[] a = StdIn.readStrings();
        Heap.sort(a);
        show(a);
    }
}