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/******************************************************************************
* Compilation: javac MergeBU.java
* Execution: java MergeBU < 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
* bottom-up mergesort.
*
* % more tiny.txt
* S O R T E X A M P L E
*
* % java MergeBU < 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 MergeBU < words3.txt
* all bad bed bug dad ... yes yet zoo [ one string per line ]
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code MergeBU} class provides static methods for sorting an
* array using bottom-up mergesort.
*
* For additional documentation, see Section 2.1 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class MergeBU {
// This class should not be instantiated.
private MergeBU() { }
// 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) {
// 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++]; // this copying is unneccessary
else if (j > hi) a[k] = aux[i++];
else if (less(aux[j], aux[i])) a[k] = aux[j++];
else a[k] = aux[i++];
}
}
/**
* Rearranges the array in ascending order, using the natural order.
* @param a the array to be sorted
*/
public static void sort(Comparable[] a) {
int n = a.length;
Comparable[] aux = new Comparable[n];
for (int len = 1; len < n; len *= 2) {
for (int lo = 0; lo < n-len; lo += len+len) {
int mid = lo+len-1;
int hi = Math.min(lo+len+len-1, n-1);
merge(a, aux, lo, mid, hi);
}
}
assert isSorted(a);
}
/***********************************************************************
* Helper sorting functions.
***************************************************************************/
// 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) {
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; bottom-up
* 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();
MergeBU.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.
******************************************************************************/