随机化快速排序(Java实现)

1 RANDOMIZED-PARTITION(A, p, r)
2 i = RANDOM(p, r)
3 exchange A[r] with A[i]
4 return PARTITION(A, p ,r)

1 RANDOMIZED-QUICKSORT(A, p, r)
2     if p < r
3         q = RANDOMIZED-PARTITION(A, p, r)
4         RANDOMIZED-QUICKSORT(A, p, q-1)
5         RANDOMIZED-QUICKSORT(A, q+1, r)

 1 package quicksort;
 2 
 3 import java.util.Random;
 4 
 5 public class RandomQuickSort {
 6 
 7     public void Sort(int[] a, int p, int r) {
 8         if (p < r) {
 9             int q = Partition(a, p, r);
10             Sort(a, p, q-1);
11             Sort(a,q+1, r);            
12         }
13     }
14     
15     private int Partition(int[] A, int p, int r) {
16         /*随机选取主元元素*/
17         Random random = new Random();
18         int random_index = random.nextInt(r-p+1)+p;
19         System.out.println("random_index="+random_index);
20         int temp = A[random_index];
21         A[random_index] = A[r];
22         A[r] = temp;
23         
24         int x = A[r];  //pivot = A[p]
25         int i = p-1;
26         for (int j = p; j < r; j++) {
27             if (A[j] <= x) {  //与pivot作比较
28                 i++;
29                 int tmp = A[j];
30                 A[j] = A[i];
31                 A[i] = tmp;
32             }
33         }
34         
35         int tmp = A[r];
36         A[r] = A[i+1];
37         A[i+1] = tmp;
38         
39         return i+1;
40         
41     }
42     
43 }

 1 package quicksort;
 2 
 3 public class RandomQuickSortTest {
 4 
 5 //    static int[] a ={2, 8, 7, 1, 3, 5, 6, 4};
 6     static int[] a ={13, 19, 9, 5, 12, 8, 7, 4, 21, 2, 6, 11};
 7     public static void main(String[] args) {
 8         RandomQuickSort randomQuickSort = new RandomQuickSort();
 9         randomQuickSort.Sort(a, 0, a.length-1);
10         
11         for (int i = 0; i < a.length; i++) {
12             System.out.print(a[i]+" ");
13         }
14     }
15 }

random_index=2
random_index=2
random_index=0
random_index=0
random_index=8
random_index=10
random_index=7
random_index=7
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