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128 lines
3.8 KiB
Java
128 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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/*************************************************************************
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* Compilation: javac Shell.java
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* Execution: java Shell < input.txt
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* Dependencies: StdOut.java StdIn.java
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* Data files: http://algs4.cs.princeton.edu/21sort/tiny.txt
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* http://algs4.cs.princeton.edu/21sort/words3.txt
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*
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* Sorts a sequence of strings from standard input using shellsort.
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*
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* Uses increment sequence proposed by Sedgewick and Incerpi.
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* The nth element of the sequence is the smallest integer >= 2.5^n
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* that is relatively prime to all previous terms in the sequence.
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* For example, incs[4] is 41 because 2.5^4 = 39.0625 and 41 is
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* the next integer that is relatively prime to 3, 7, and 16.
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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 Shell < 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 Shell < 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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/**
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* The Shell class provides static methods for sorting an
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* array using Shellsort with Knuth's increment sequence (1, 4, 13, 40, ...).
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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 Shell {
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// This class should not be instantiated.
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private Shell() { }
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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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int N = a.length;
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// 3x+1 increment sequence: 1, 4, 13, 40, 121, 364, 1093, ...
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int h = 1;
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while (h < N/3) h = 3*h + 1;
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while (h >= 1) {
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// h-sort the array
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for (int i = h; i < N; i++) {
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for (int j = i; j >= h && less(a[j], a[j-h]); j -= h) {
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exch(a, j, j-h);
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}
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}
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assert isHsorted(a, h);
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h /= 3;
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}
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assert isSorted(a);
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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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// 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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for (int i = 1; i < a.length; 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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// is the array h-sorted?
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private static boolean isHsorted(Comparable[] a, int h) {
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for (int i = h; i < a.length; i++)
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if (less(a[i], a[i-h])) 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; Shellsorts them;
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* 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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Shell.sort(a);
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show(a);
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}
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}
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