edu.princeton.cs.algs4.Quick Maven / Gradle / Ivy
/******************************************************************************
* Compilation: javac Quick.java
* Execution: java Quick < 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 quicksort.
*
* % more tiny.txt
* S O R T E X A M P L E
*
* % java Quick < 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 Quick < words3.txt
* all bad bed bug dad ... yes yet zoo [ one string per line ]
*
*
* Remark: For a type-safe version that uses static generics, see
*
* https://algs4.cs.princeton.edu/23quicksort/QuickPedantic.java
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code Quick} class provides static methods for sorting an
* array and selecting the ith smallest element in an array using quicksort.
*
* 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 Quick {
// This class should not be instantiated.
private Quick() { }
/**
* 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;
int j = partition(a, lo, hi);
sort(a, lo, j-1);
sort(a, j+1, hi);
assert isSorted(a, lo, 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 i = lo;
int j = hi + 1;
Comparable v = a[lo];
while (true) {
// find item on lo to swap
while (less(a[++i], v)) {
if (i == hi) break;
}
// find item on hi to swap
while (less(v, a[--j])) {
if (j == lo) break; // redundant since a[lo] acts as sentinel
}
// check if pointers cross
if (i >= j) break;
exch(a, i, 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;
}
/**
* Rearranges the array so that {@code a[k]} contains the kth smallest key;
* {@code a[0]} through {@code a[k-1]} are less than (or equal to) {@code a[k]}; and
* {@code a[k+1]} through {@code a[n-1]} are greater than (or equal to) {@code a[k]}.
*
* @param a the array
* @param k the rank of the key
* @return the key of rank {@code k}
* @throws IllegalArgumentException unless {@code 0 <= k < a.length}
*/
public static Comparable select(Comparable[] a, int k) {
if (k < 0 || k >= a.length) {
throw new IllegalArgumentException("index is not between 0 and " + a.length + ": " + k);
}
StdRandom.shuffle(a);
int lo = 0, hi = a.length - 1;
while (hi > lo) {
int i = partition(a, lo, hi);
if (i > k) hi = i - 1;
else if (i < k) lo = i + 1;
else return a[i];
}
return a[lo];
}
/***************************************************************************
* Helper sorting functions.
***************************************************************************/
// is v < w ?
private static boolean less(Comparable v, Comparable w) {
if (v == w) return false; // optimization when reference equals
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; quicksorts them;
* and prints them to standard output in ascending order.
* Shuffles the array and then prints the strings again to
* standard output, but this time, using the select method.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
String[] a = StdIn.readAllStrings();
Quick.sort(a);
show(a);
assert isSorted(a);
// shuffle
StdRandom.shuffle(a);
// display results again using select
StdOut.println();
for (int i = 0; i < a.length; i++) {
String ith = (String) Quick.select(a, i);
StdOut.println(ith);
}
}
}
/******************************************************************************
* 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.
******************************************************************************/