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125 lines
3.8 KiB
Java
125 lines
3.8 KiB
Java
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import edu.princeton.cs.introcs.StdIn;
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import edu.princeton.cs.introcs.StdOut;
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/*************************************************************************
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* Compilation: javac MergeBU.java
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* Execution: java MergeBU < input.txt
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* Dependencies: StdOut.java StdIn.java
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* Data files: http://algs4.cs.princeton.edu/22mergesort/tiny.txt
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* http://algs4.cs.princeton.edu/22mergesort/words3.txt
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*
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* Sorts a sequence of strings from standard input using
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* bottom-up mergesort.
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*
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* % more tiny.txt
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* S O R T E X A M P L E
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*
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* % java MergeBU < tiny.txt
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* A E E L M O P R S T X [ one string per line ]
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*
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* % more words3.txt
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* bed bug dad yes zoo ... all bad yet
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*
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* % java MergeBU < words3.txt
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* all bad bed bug dad ... yes yet zoo [ one string per line ]
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*
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*************************************************************************/
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/**
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* The MergeBU class provides static methods for sorting an
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* array using bottom-up mergesort.
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*
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* For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
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* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
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*
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* @author Robert Sedgewick
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* @author Kevin Wayne
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*/
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public class MergeBU {
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// This class should not be instantiated.
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private MergeBU() { }
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// stably merge a[lo..mid] with a[mid+1..hi] using aux[lo..hi]
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private static void merge(Comparable[] a, Comparable[] aux, int lo, int mid, int hi) {
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// copy to aux[]
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for (int k = lo; k <= hi; k++) {
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aux[k] = a[k];
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}
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// merge back to a[]
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int i = lo, j = mid+1;
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for (int k = lo; k <= hi; k++) {
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if (i > mid) a[k] = aux[j++]; // this copying is unneccessary
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else if (j > hi) a[k] = aux[i++];
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else if (less(aux[j], aux[i])) a[k] = aux[j++];
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else a[k] = aux[i++];
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}
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}
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/**
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* Rearranges the array in ascending order, using the natural order.
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* @param a the array to be sorted
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*/
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public static void sort(Comparable[] a) {
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int N = a.length;
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Comparable[] aux = new Comparable[N];
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for (int n = 1; n < N; n = n+n) {
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for (int i = 0; i < N-n; i += n+n) {
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int lo = i;
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int m = i+n-1;
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int hi = Math.min(i+n+n-1, N-1);
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merge(a, aux, lo, m, hi);
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}
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}
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assert isSorted(a);
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}
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/***********************************************************************
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* Helper sorting functions
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***********************************************************************/
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// is v < w ?
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private static boolean less(Comparable v, Comparable w) {
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return (v.compareTo(w) < 0);
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}
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// exchange a[i] and a[j]
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private static void exch(Object[] a, int i, int j) {
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Object swap = a[i];
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a[i] = a[j];
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a[j] = swap;
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}
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/***********************************************************************
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* Check if array is sorted - useful for debugging
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***********************************************************************/
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private static boolean isSorted(Comparable[] a) {
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for (int i = 1; i < a.length; i++)
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if (less(a[i], a[i-1])) return false;
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return true;
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}
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// print array to standard output
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private static void show(Comparable[] a) {
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for (int i = 0; i < a.length; i++) {
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StdOut.println(a[i]);
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}
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}
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/**
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* Reads in a sequence of strings from standard input; bottom-up
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* mergesorts them; and prints them to standard output in ascending order.
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*/
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public static void main(String[] args) {
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String[] a = StdIn.readAllStrings();
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MergeBU.sort(a);
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show(a);
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}
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}
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