快速排序

 1 package sorts;
 2 
 3 import java.util.Arrays;
 4 import java.util.Random;
 5 
 6 public class QuickSort {
 7     public static <T extends Comparable<T>> 
 8     void sort(T[] a, int low, int high) {
 9         if (low < high) {
10             int mid = partition(a, low, high);
11             sort(a, low, mid - 1);
12             sort(a, mid + 1, high);
13         }
14     }
15     private static <T extends Comparable<T>> 
16     int partition(T[] a, int low, int high) {
17         median_of_three(a, low, high);
18         T pivot = a[low]; // low is the median of three, as the pivot
19         int i = low - 1;
20         for (int j = low + 1; j <= high; ++j) {
21             if (a[j].compareTo(pivot)<0) {
22                 ++i;
23                 swap(a, i, j);
24             }
25         }
26         a[i+1] = pivot;
27         return i+1;
28     }
29     // set the median of low, high, mid to low
30     private static <T extends Comparable<T>> 
31     void median_of_three(T[] a, int low, int high) {
32         int mid = low + ((high-low)>>1); // (high - low) / 2
33         if (a[mid].compareTo(a[high])>0) {
34             swap(a, mid, high);
35         }
36         if (a[low].compareTo(a[high])>0) {
37             swap(a, low, high);
38         }
39         if (a[mid].compareTo(a[low])>0) {
40             swap(a, mid, low);
41         }
42         // a[mid] <= a[low] <= a[high], low is the median
43     }
44     private static <T> void swap(T[] a, int index1, int index2) {
45         T tmp = a[index1];
46         a[index1] = a[index2];
47         a[index2] = tmp;
48     }
49     
50     // test
51     public static void main(String[] args) {
52         Random random = new Random();
53         int num = 10;
54         int bound = 100;
55         Integer[] a = new Integer[num];
56         for (int i = 0; i < num; i++) {
57             a[i] = random.nextInt(bound);
58         }
59         QuickSort.sort(a, 0, a.length-1);
60         System.out.println(Arrays.toString(a));
61     }
62 }