import edu.princeton.cs.introcs.StdIn; import edu.princeton.cs.introcs.StdOut; import edu.princeton.cs.introcs.StdRandom; /************************************************************************* * Compilation: javac Quick3way.java * Execution: java Quick3way < input.txt * Dependencies: StdOut.java StdIn.java * Data files: http://algs4.cs.princeton.edu/23quicksort/tiny.txt * http://algs4.cs.princeton.edu/23quicksort/words3.txt * * Sorts a sequence of strings from standard input using 3-way quicksort. * * % more tiny.txt * S O R T E X A M P L E * * % java Quick3way < tiny.txt * A E E L M O P R S T X [ one string per line ] * * % more words3.txt * bed bug dad yes zoo ... all bad yet * * % java Quick3way < words3.txt * all bad bed bug dad ... yes yet zoo [ one string per line ] * *************************************************************************/ /** * The Quick3way class provides static methods for sorting an * array using quicksort with 3-way partitioning. * * For additional documentation, see Section 2.1 of * Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne. * * @author Robert Sedgewick * @author Kevin Wayne */ public class Quick3way { // This class should not be instantiated. private Quick3way() { } /** * Rearranges the array in ascending order, using the natural order. * @param a the array to be sorted */ public static void sort(Comparable[] a) { StdRandom.shuffle(a); sort(a, 0, a.length - 1); assert isSorted(a); } // quicksort the subarray a[lo .. hi] using 3-way partitioning private static void sort(Comparable[] a, int lo, int hi) { if (hi <= lo) return; int lt = lo, gt = hi; Comparable v = a[lo]; int i = lo; while (i <= gt) { int cmp = a[i].compareTo(v); if (cmp < 0) exch(a, lt++, i++); else if (cmp > 0) exch(a, i, gt--); else i++; } // a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi]. sort(a, lo, lt-1); sort(a, gt+1, hi); assert isSorted(a, lo, hi); } /*********************************************************************** * Helper sorting functions ***********************************************************************/ // is v < w ? private static boolean less(Comparable v, Comparable w) { return (v.compareTo(w) < 0); } // does v == w ? private static boolean eq(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) { return isSorted(a, 0, a.length - 1); } private static boolean isSorted(Comparable[] a, int lo, int hi) { for (int i = lo + 1; i <= hi; 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; 3-way * quicksorts them; and prints them to standard output in ascending order. */ public static void main(String[] args) { String[] a = StdIn.readAllStrings(); Quick3way.sort(a); show(a); } }