QuickKR.java

Below is the syntax highlighted version of QuickKR.java from §2.3 Quicksort.

/******************************************************************************
 *  Compilation:  javac QuickKR.java
 *  Execution:    java QuickKR N
 *  Dependencies: StdIn.java StdOut.java
 *
 *  Reference: The C Programming Language by Brian W. Kernighan and
 *  Dennis M. Ritchie, p. 87.
 *
 *  Warning: goes quadratic if many duplicate keys.
 *
 ******************************************************************************/

public class QuickKR {

    public static void sort(Comparable[] a) {
        sort(a, 0, a.length - 1);
    }

    private static void sort(Comparable[] a, int lo, int hi) {
        if (hi <= lo) return;
        int mid = lo + (hi - lo) / 2;
        exch(a, lo, mid);  // use middle element as partition
        int last = lo;
        for (int i = lo + 1; i <= hi; i++)
            if (less(a[i], a[lo])) exch(a, ++last, i);
        exch(a, lo, last);
        sort(a, lo, last-1);
        sort(a, last+1, hi);
    }


   /***************************************************************************
    *  Helper sorting functions.
    ***************************************************************************/

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

    // exchange a[i] and a[j]
    private static void exch(Object[] a, int i, int j) {
        Object swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }


   /***************************************************************************
    *  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]);
        }
    }


    /**
     * Reads in a sequence of strings from standard input; quicksorts them;
     * and prints them to standard output in ascending order.
     * Shuffles the array and then prints the strings again to
     * standard output, but this time, using the select method.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        QuickKR.sort(a);
        show(a);
        assert isSorted(a);
    }
}