edu.princeton.cs.algs4.QuickX Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac QuickX.java
* Execution: java QuickX < 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
*
* Uses the Hoare's 2-way partitioning scheme, chooses the partitioning
* element using median-of-3, and cuts off to insertion sort.
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code QuickX} class provides static methods for sorting an array
* using an optimized version of quicksort (using Hoare's 2-way partitioning
* algorithm, median-of-3 to choose the partitioning element, and cutoff
* to insertion sort).
*
* 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 QuickX {
// cutoff to insertion sort, must be >= 1
private static final int INSERTION_SORT_CUTOFF = 8;
// This class should not be instantiated.
private QuickX() { }
/**
* 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 from a[lo] to a[hi]
private static void sort(Comparable[] a, int lo, int hi) {
if (hi <= lo) return;
// cutoff to insertion sort (Insertion.sort() uses half-open intervals)
int n = hi - lo + 1;
if (n <= INSERTION_SORT_CUTOFF) {
Insertion.sort(a, lo, hi + 1);
return;
}
int j = partition(a, lo, hi);
sort(a, lo, j-1);
sort(a, j+1, hi);
}
// partition the subarray a[lo..hi] so that a[lo..j-1] <= a[j] <= a[j+1..hi]
// and return the index j.
private static int partition(Comparable[] a, int lo, int hi) {
int n = hi - lo + 1;
int m = median3(a, lo, lo + n/2, hi);
exch(a, m, lo);
int i = lo;
int j = hi + 1;
Comparable v = a[lo];
// a[lo] is unique largest element
while (less(a[++i], v)) {
if (i == hi) { exch(a, lo, hi); return hi; }
}
// a[lo] is unique smallest element
while (less(v, a[--j])) {
if (j == lo + 1) return lo;
}
// the main loop
while (i < j) {
exch(a, i, j);
while (less(a[++i], v)) ;
while (less(v, a[--j])) ;
}
// put partitioning item v at a[j]
exch(a, lo, j);
// now, a[lo .. j-1] <= a[j] <= a[j+1 .. hi]
return j;
}
// return the index of the median element among a[i], a[j], and a[k]
private static int median3(Comparable[] a, int i, int j, int k) {
return (less(a[i], a[j]) ?
(less(a[j], a[k]) ? j : less(a[i], a[k]) ? k : i) :
(less(a[k], a[j]) ? j : less(a[k], a[i]) ? k : i));
}
/***************************************************************************
* 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
* (using an optimized version of 2-way quicksort);
* 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();
QuickX.sort(a);
assert isSorted(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.
******************************************************************************/