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131 lines
3.8 KiB
Java
131 lines
3.8 KiB
Java
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import edu.princeton.cs.introcs.StdIn;
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import edu.princeton.cs.introcs.StdOut;
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import edu.princeton.cs.introcs.StdRandom;
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/*************************************************************************
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* Compilation: javac Quick3way.java
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* Execution: java Quick3way < input.txt
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* Dependencies: StdOut.java StdIn.java
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* Data files: http://algs4.cs.princeton.edu/23quicksort/tiny.txt
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* http://algs4.cs.princeton.edu/23quicksort/words3.txt
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*
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* Sorts a sequence of strings from standard input using 3-way quicksort.
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*
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* % more tiny.txt
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* S O R T E X A M P L E
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*
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* % java Quick3way < tiny.txt
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* A E E L M O P R S T X [ one string per line ]
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*
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* % more words3.txt
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* bed bug dad yes zoo ... all bad yet
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*
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* % java Quick3way < words3.txt
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* all bad bed bug dad ... yes yet zoo [ one string per line ]
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*
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*************************************************************************/
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/**
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* The Quick3way class provides static methods for sorting an
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* array using quicksort with 3-way partitioning.
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*
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* For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
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* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
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*
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* @author Robert Sedgewick
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* @author Kevin Wayne
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*/
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public class Quick3way {
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// This class should not be instantiated.
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private Quick3way() { }
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/**
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* Rearranges the array in ascending order, using the natural order.
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* @param a the array to be sorted
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*/
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public static void sort(Comparable[] a) {
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StdRandom.shuffle(a);
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sort(a, 0, a.length - 1);
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assert isSorted(a);
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}
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// quicksort the subarray a[lo .. hi] using 3-way partitioning
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private static void sort(Comparable[] a, int lo, int hi) {
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if (hi <= lo) return;
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int lt = lo, gt = hi;
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Comparable v = a[lo];
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int i = lo;
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while (i <= gt) {
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int cmp = a[i].compareTo(v);
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if (cmp < 0) exch(a, lt++, i++);
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else if (cmp > 0) exch(a, i, gt--);
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else i++;
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}
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// a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi].
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sort(a, lo, lt-1);
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sort(a, gt+1, hi);
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assert isSorted(a, lo, hi);
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}
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/***********************************************************************
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* Helper sorting functions
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***********************************************************************/
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// is v < w ?
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private static boolean less(Comparable v, Comparable w) {
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return (v.compareTo(w) < 0);
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}
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// does v == w ?
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private static boolean eq(Comparable v, Comparable w) {
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return (v.compareTo(w) == 0);
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}
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// exchange a[i] and a[j]
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private static void exch(Object[] a, int i, int j) {
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Object swap = a[i];
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a[i] = a[j];
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a[j] = swap;
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}
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/***********************************************************************
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* Check if array is sorted - useful for debugging
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***********************************************************************/
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private static boolean isSorted(Comparable[] a) {
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return isSorted(a, 0, a.length - 1);
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}
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private static boolean isSorted(Comparable[] a, int lo, int hi) {
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for (int i = lo + 1; i <= hi; i++)
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if (less(a[i], a[i-1])) return false;
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return true;
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}
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// print array to standard output
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private static void show(Comparable[] a) {
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for (int i = 0; i < a.length; i++) {
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StdOut.println(a[i]);
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}
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}
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/**
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* Reads in a sequence of strings from standard input; 3-way
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* quicksorts them; and prints them to standard output in ascending order.
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*/
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public static void main(String[] args) {
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String[] a = StdIn.readAllStrings();
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Quick3way.sort(a);
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show(a);
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}
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}
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