Below is the syntax highlighted version of Merge.java.
/****************************************************************************** * Compilation: javac Merge.java * Execution: java Merge < input.txt * Dependencies: StdOut.java StdIn.java * Data files: https://algs4.cs.princeton.edu/22mergesort/tiny.txt * https://algs4.cs.princeton.edu/22mergesort/words3.txt * * Sorts a sequence of strings from standard input using mergesort. * * % more tiny.txt * S O R T E X A M P L E * * % java Merge < 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 Merge < words3.txt * all bad bed bug dad ... yes yet zoo [ one string per line ] * ******************************************************************************/ package edu.princeton.cs.algs4; /** * The {@code Merge} class provides static methods for sorting an * array using a top-down, recursive version of <em>mergesort</em>. * <p> * This implementation takes Θ(<em>n</em> log <em>n</em>) time * to sort any array of length <em>n</em> (assuming comparisons * take constant time). It makes between * ~ ½ <em>n</em> log<sub>2</sub> <em>n</em> and * ~ 1 <em>n</em> log<sub>2</sub> <em>n</em> compares. * <p> * This sorting algorithm is stable. * It uses Θ(<em>n</em>) extra memory (not including the input array). * <p> * For additional documentation, see * <a href="https://algs4.cs.princeton.edu/22mergesort">Section 2.2</a> of * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne. * For an optimized version, see {@link MergeX}. * * @author Robert Sedgewick * @author Kevin Wayne */ public class Merge { // This class should not be instantiated. private Merge() { } // stably merge a[lo .. mid] with a[mid+1 ..hi] using aux[lo .. hi] private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) { // precondition: a[lo .. mid] and a[mid+1 .. hi] are sorted subarrays assert isSorted(a, lo, mid); assert isSorted(a, mid+1, hi); // copy to aux[] for (int k = lo; k <= hi; k++) { aux[k] = a[k]; } // merge back to a[] int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) a[k] = aux[j++]; else if (j > hi) a[k] = aux[i++]; else if (less(aux[j], aux[i])) a[k] = aux[j++]; else a[k] = aux[i++]; } // postcondition: a[lo .. hi] is sorted assert isSorted(a, lo, hi); } // mergesort a[lo..hi] using auxiliary array aux[lo..hi] private static void sort(Comparable[] a, Comparable[] aux, int lo, int hi) { if (hi <= lo) return; int mid = lo + (hi - lo) / 2; sort(a, aux, lo, mid); sort(a, aux, mid + 1, hi); merge(a, aux, lo, mid, hi); } /** * Rearranges the array in ascending order, using the natural order. * @param a the array to be sorted */ public static void sort(Comparable[] a) { Comparable[] aux = new Comparable[a.length]; sort(a, aux, 0, a.length-1); assert isSorted(a); } /*************************************************************************** * Helper sorting function. ***************************************************************************/ // is v < w ? private static boolean less(Comparable v, Comparable w) { return v.compareTo(w) < 0; } /*************************************************************************** * 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; } /*************************************************************************** * Index mergesort. ***************************************************************************/ // stably merge a[lo .. mid] with a[mid+1 .. hi] using aux[lo .. hi] private static void merge(Comparable[] a, int[] index, int[] aux, int lo, int mid, int hi) { // copy to aux[] for (int k = lo; k <= hi; k++) { aux[k] = index[k]; } // merge back to a[] int i = lo, j = mid+1; for (int k = lo; k <= hi; k++) { if (i > mid) index[k] = aux[j++]; else if (j > hi) index[k] = aux[i++]; else if (less(a[aux[j]], a[aux[i]])) index[k] = aux[j++]; else index[k] = aux[i++]; } } /** * Returns a permutation that gives the elements in the array in ascending order. * @param a the array * @return a permutation {@code p[]} such that {@code a[p[0]]}, {@code a[p[1]]}, * ..., {@code a[p[n-1]]} are in ascending order */ public static int[] indexSort(Comparable[] a) { int n = a.length; int[] index = new int[n]; for (int i = 0; i < n; i++) index[i] = i; int[] aux = new int[n]; sort(a, index, aux, 0, n-1); return index; } // mergesort a[lo..hi] using auxiliary array aux[lo..hi] private static void sort(Comparable[] a, int[] index, int[] aux, int lo, int hi) { if (hi <= lo) return; int mid = lo + (hi - lo) / 2; sort(a, index, aux, lo, mid); sort(a, index, aux, mid + 1, hi); merge(a, index, aux, lo, mid, hi); } // 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; mergesorts 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(); Merge.sort(a); show(a); } } /****************************************************************************** * Copyright 2002-2022, 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. ******************************************************************************/