quickselect java_Java QuickSelect

Java QuickSelect

/**

*

*

*

Copyright 1994-2018 JasonInternational

*

All rights reserved.

*

Created on 2018年4月10日 上午9:46:32

*

Created by Jason

*

*

*/

package cn.ucaner.algorithm.search;

import java.util.Random;

/**

* In computer science, quickselect is a selection algorithm to find the k-th smallest element in an unordered list. It is related to the quicksort sorting algorithm.

*

* Worst-case performance О(n2)

* Best-case performance О(n)

* Average performance O(n)

*

* @see Quickselect (Wikipedia)

*

* @author Justin Wetherell

*/

public class QuickSelect {

private static final Random RANDOM = new Random();

private static int[] unsorted = null;

private static int[] temp = null;

public static final int find(int value, int[] array) {

unsorted = array;

temp = new int[unsorted.length];

try {

int tempLength = unsorted.length;

int length = tempLength;

int pivot = unsorted[0];

while (length > 0) {

length = tempLength;

pivot = unsorted[RANDOM.nextInt(length)];

tempLength = 0;

for (int i = 0; i < length; i++) {

int iValue = unsorted[i];

if (value == iValue)

return i;

else if (value > pivot && iValue > pivot)

temp[tempLength++] = iValue;

else if (value < pivot && iValue < pivot)

temp[tempLength++] = iValue;

}

unsorted = temp;

length = tempLength;

}

return Integer.MAX_VALUE;

} finally {

QuickSelect.unsorted = null;

QuickSelect.temp = null;

}

}

}