随机快速排序在Java中需要suble解决? [英] randomized quick sort in Java need a suble fix?
问题描述
我已经实现了快速排序
算法,使用列表作为支点的第一个元素,它工作得很好。我现在重构
来选择一个随机指数作为主元,交换的第一个元素,并做快速排序子程序。不知何故,这是行不通的,我没有得到的排序的数组。
这里是我的code,这是不言自明,但我很高兴,如果任何疑问需要解释。
I have implemented the quicksort
algorithm that uses the first element of the list as pivot and it worked fine. now I refactored
to pick a random index as pivot element, swap with the first element and do the quicksort subroutine. somehow, it does not work, I do not get the sorted array.
here is my code, which is self-explanatory, but I am happy to explain if any clarification needed.
public class Qsort {
public static void quickSort2(int[] arr, int i, int j){
if (i<j){
int part =randPartition(arr, i, j);
quickSort2(arr, i, part-1);
quickSort2(arr, part+1, j);
}
}
public static int randPartition(int[] arr, int start, int end){
//int pivId = (int)(Math.random()*(end-start)+1)+start; -- *****
int pivId = start;
//System.out.println("pivId is "+ pivId + "; start is " + start + "; end is " + end);
System.out.println("arr is "+ Arrays.toString(arr));
int pivot = arr[pivId];
swap(arr, start, pivId);
int i = start;
for (int j = start+1;j<=end;j++){
if (arr[j]<pivot){
i++;
swap(arr,i,j);
}
}
swap(arr, start,i);
return i;
}
private static void swap(int[] arr, int index1, int index2) {
int temp = arr[index1];
arr[index1]=arr[index2];
arr[index2]=temp;
}
public static void main(String[] args) {
int[] data2 = new int[]{10,11,9,7,5};
System.out.println("Unsorted array data " + Arrays.toString(data2));
quickSort2(data2, 0, data.length-1);
System.out.println("Sorted array data " + Arrays.toString(data2));
}
}
我已经注释掉随机支点计算在code与*****
。我没有看到任何问题,但具有随机支点计算破坏了code
I have commented out the random pivot calculation in the code with *****
. I dont see any problem with it, but having the random pivot calculation destroys the code
推荐答案
你的code,其余是确定的,但
the rest of your code is OK,but
int pivId = (int)(Math.random()*(end-start)+1)+start; -- *****
有一些问题。 考虑结束==开始,然后你长了pivID == 1 +启动。
has some problems. Consider end == start, then u got pivID == 1+start.
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