1 /******************************************************************************
2 * Compilation: javac Selection.java
3 * Execution: java Selection < input.txt
4 * Dependencies: StdOut.java StdIn.java
5 * Data files: http://algs4.cs.princeton.edu/21elementary/tiny.txt
6 * http://algs4.cs.princeton.edu/21elementary/words3.txt
7 *
8 * Sorts a sequence of strings from standard input using selection sort.
9 *
10 * % more tiny.txt
11 * S O R T E X A M P L E
12 *
13 * % java Selection < tiny.txt
14 * A E E L M O P R S T X [ one string per line ]
15 *
16 * % more words3.txt
17 * bed bug dad yes zoo ... all bad yet
18 *
19 * % java Selection < words3.txt
20 * all bad bed bug dad ... yes yet zoo [ one string per line ]
21 *
22 ******************************************************************************/
23
24 package edu.princeton.cs.algs4;
25
26 import java.util.Comparator;
27
28 /**
29 * The {@code Selection} class provides static methods for sorting an
30 * array using selection sort.
31 * <p>
32 * For additional documentation, see <a href="http://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
33 * <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
34 *
35 * @author Robert Sedgewick
36 * @author Kevin Wayne
37 */
38 public class Selection {
39
40 // This class should not be instantiated.
41 private Selection() { }
42
43 /**
44 * Rearranges the array in ascending order, using the natural order.
45 * @param a the array to be sorted
46 */
47 public static void sort(Comparable[] a) {//因为字符串实现了comparable接口故可以使用comparable作为父类 是多态的体现
48 int n = a.length;
49 for (int i = 0; i < n; i++) {
50 int min = i;
51 for (int j = i+1; j < n; j++) {
52 if (less(a[j], a[min])) min = j;
53 }
54 exch(a, i, min);
55 assert isSorted(a, 0, i);
56 }
57 assert isSorted(a);
58 }
59
60 /**
61 * Rearranges the array in ascending order, using a comparator.
62 * @param a the array
63 * @param comparator the comparator specifying the order
64 */
65 public static void sort(Object[] a, Comparator comparator) {
66 int n = a.length;
67 for (int i = 0; i < n; i++) {
68 int min = i;
69 for (int j = i+1; j < n; j++) {
70 if (less(comparator, a[j], a[min])) min = j;
71 }
72 exch(a, i, min);
73 assert isSorted(a, comparator, 0, i);
74 }
75 assert isSorted(a, comparator);
76 }
77
78
79 /***************************************************************************
80 * Helper sorting functions.
81 ***************************************************************************/
82
83 // is v < w ?
84 private static boolean less(Comparable v, Comparable w) {
85 return v.compareTo(w) < 0;//若v小于w则返回true
86 }
87
88 // is v < w ?
89 private static boolean less(Comparator comparator, Object v, Object w) {
90 return comparator.compare(v, w) < 0;
91 }
92
93
94 // exchange a[i] and a[j]
95 private static void exch(Object[] a, int i, int j) {
96 Object swap = a[i];
97 a[i] = a[j];
98 a[j] = swap;
99 }
100
101
102 /***************************************************************************
103 * Check if array is sorted - useful for debugging.
104 ***************************************************************************/
105
106 // is the array a[] sorted?
107 private static boolean isSorted(Comparable[] a) {
108 return isSorted(a, 0, a.length - 1);
109 }
110
111 // is the array sorted from a[lo] to a[hi]
112 private static boolean isSorted(Comparable[] a, int lo, int hi) {
113 for (int i = lo + 1; i <= hi; i++)
114 if (less(a[i], a[i-1])) return false;
115 return true;
116 }
117
118 // is the array a[] sorted?
119 private static boolean isSorted(Object[] a, Comparator comparator) {
120 return isSorted(a, comparator, 0, a.length - 1);
121 }
122
123 // is the array sorted from a[lo] to a[hi]
124 private static boolean isSorted(Object[] a, Comparator comparator, int lo, int hi) {
125 for (int i = lo + 1; i <= hi; i++)
126 if (less(comparator, a[i], a[i-1])) return false;
127 return true;
128 }
129
130
131
132 // print array to standard output
133 private static void show(Comparable[] a) {
134 for (int i = 0; i < a.length; i++) {
135 StdOut.print(a[i]);
136 }
137 }
138
139 /**
140 * Reads in a sequence of strings from standard input; selection sorts them;
141 * and prints them to standard output in ascending order.
142 *
143 * @param args the command-line arguments
144 */
145 public static void main(String[] args) {
146 String[] a = StdIn.readAllStrings();//使用输入流的方式进行输入，当结束时应该按住ctrl+c结束输入流
147 show(a);
148 System.out.println();
149 Selection.sort(a);
150 show(a);
151 }
152 }
153
154 /******************************************************************************
155 * Copyright 2002-2016, Robert Sedgewick and Kevin Wayne.
156 *
157 * This file is part of algs4.jar, which accompanies the textbook
158 *
159 * Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
160 * Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
161 * http://algs4.cs.princeton.edu
162 *
163 *
164 * algs4.jar is free software: you can redistribute it and/or modify
165 * it under the terms of the GNU General Public License as published by
166 * the Free Software Foundation, either version 3 of the License, or
167 * (at your option) any later version.
168 *
169 * algs4.jar is distributed in the hope that it will be useful,
170 * but WITHOUT ANY WARRANTY; without even the implied warranty of
171 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
172 * GNU General Public License for more details.
173 *
174 * You should have received a copy of the GNU General Public License
175 * along with algs4.jar. If not, see http://www.gnu.org/licenses.
176 ******************************************************************************/
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