edu.princeton.cs.algs4.Quick3way Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac Quick3way.java
* Execution: java Quick3way < input.txt
* Dependencies: StdOut.java StdIn.java
* Data files: https://algs4.cs.princeton.edu/23quicksort/tiny.txt
* https://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 ]
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code Quick3way} class provides static methods for sorting an
* array using quicksort with 3-way partitioning.
*
* For additional documentation,
* see Section 2.3 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 + 1;
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;
}
// 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.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
String[] a = StdIn.readAllStrings();
Quick3way.sort(a);
show(a);
}
}
/******************************************************************************
* Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/