计数排序

 1 package sorts;
 2 
 3 import java.util.Arrays;
 4 import java.util.Random;
 5 
 6 public class CountingSort {
 7     /**
 8      * @param a the target array
 9      * @param b the array to store the result of the sort
10      * @param k in the target array, each element is in the range [0, k]
11      * */
12     public static void sort(int[] a, int[] b, int k) {
13         int[] c = new int[k+1]; // counting array
14         for (int i = 0; i < k; i++) {
15             c[i] = 0;
16         }
17         for (int i = 0; i < a.length; i++) {
18             ++c[a[i]]; // c[x] is the number of elements in ‘a‘ equal to x.
19         }
20         for (int i = 1; i <= k; i++) {
21             c[i] = c[i] + c[i-1]; // c[x] is the number of elements in ‘a‘ no more than x.
22         }
23         for (int i = a.length - 1; i >= 0; i--) {
24             b[c[a[i]]-1] = a[i]; // convert to index, must subtract 1
25             --c[a[i]];
26         }
27     }
28     
29     // test
30     public static void main(String[] args) {
31         Random random = new Random();
32         int num = 10;
33         int bound = 100;
34         int[] a = new int[num];
35         int[] b = new int[num];
36         for (int i = 0; i < num; i++) {
37             a[i] = random.nextInt(bound);
38         }
39         CountingSort.sort(a, b, bound-1);
40         System.out.println(Arrays.toString(b));
41     }
42 }