150 lines
4.7 KiB
Java
150 lines
4.7 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 MergeX.java
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* Execution: java MergeX < 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 an
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* optimized version of 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 MergeX < 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 MergeX < 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 MergeX class provides static methods for sorting an
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* array using an optimized version of mergesort.
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*
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* For additional documentation, see <a href="http://algs4.cs.princeton.edu/22mergesort">Section 2.2</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 MergeX {
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private static final int CUTOFF = 7; // cutoff to insertion sort
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// This class should not be instantiated.
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private MergeX() { }
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private static void merge(Comparable[] src, Comparable[] dst, int lo, int mid, int hi) {
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// precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays
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assert isSorted(src, lo, mid);
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assert isSorted(src, mid+1, hi);
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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) dst[k] = src[j++];
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else if (j > hi) dst[k] = src[i++];
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else if (less(src[j], src[i])) dst[k] = src[j++]; // to ensure stability
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else dst[k] = src[i++];
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}
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// postcondition: dst[lo .. hi] is sorted subarray
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assert isSorted(dst, lo, hi);
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}
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private static void sort(Comparable[] src, Comparable[] dst, int lo, int hi) {
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// if (hi <= lo) return;
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if (hi <= lo + CUTOFF) {
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insertionSort(dst, lo, hi);
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return;
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}
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int mid = lo + (hi - lo) / 2;
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sort(dst, src, lo, mid);
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sort(dst, src, mid+1, hi);
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// if (!less(src[mid+1], src[mid])) {
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// for (int i = lo; i <= hi; i++) dst[i] = src[i];
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// return;
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// }
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// using System.arraycopy() is a bit faster than the above loop
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if (!less(src[mid+1], src[mid])) {
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System.arraycopy(src, lo, dst, lo, hi - lo + 1);
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return;
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}
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merge(src, dst, lo, mid, hi);
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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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Comparable[] aux = a.clone();
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sort(aux, a, 0, a.length-1);
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assert isSorted(a);
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}
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// sort from a[lo] to a[hi] using insertion sort
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private static void insertionSort(Comparable[] a, int lo, int hi) {
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for (int i = lo; i <= hi; i++)
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for (int j = i; j > lo && less(a[j], a[j-1]); j--)
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exch(a, j, j-1);
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}
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// exchange a[i] and a[j]
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private static void exch(Comparable[] a, int i, int j) {
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Comparable 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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// is a[i] < a[j]?
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private static boolean less(Comparable a, Comparable b) {
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return (a.compareTo(b) < 0);
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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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return isSorted(a, 0, a.length - 1);
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}
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private static boolean isSorted(Comparable[] a, int lo, int hi) {
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for (int i = lo + 1; i <= hi; 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; mergesorts them
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* (using an optimized version of mergesort);
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* 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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MergeX.sort(a);
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show(a);
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}
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}
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