53 lines
1.7 KiB
Java
53 lines
1.7 KiB
Java
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package com.jwetherell.algorithms.search;
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import java.util.Random;
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/**
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* 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.
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* <p>
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* Worst-case performance О(n2)<br>
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* Best-case performance О(n)<br>
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* Average performance O(n)<br>
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* <br>
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* https://en.wikipedia.org/wiki/Quickselect
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*
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* @author Justin Wetherell <phishman3579@gmail.com>
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*/
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public class QuickSelect {
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private static final Random RANDOM = new Random();
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private static int[] unsorted = null;
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private static int[] temp = null;
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public static final int find(int value, int[] array) {
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unsorted = array;
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temp = new int[unsorted.length];
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try {
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int tempLength = unsorted.length;
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int length = tempLength;
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int pivot = unsorted[0];
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while (length > 0) {
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length = tempLength;
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pivot = unsorted[RANDOM.nextInt(length)];
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tempLength = 0;
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for (int i = 0; i < length; i++) {
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int iValue = unsorted[i];
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if (value == iValue)
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return i;
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else if (value > pivot && iValue > pivot)
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temp[tempLength++] = iValue;
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else if (value < pivot && iValue < pivot)
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temp[tempLength++] = iValue;
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}
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unsorted = temp;
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length = tempLength;
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}
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return Integer.MAX_VALUE;
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} finally {
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QuickSelect.unsorted = null;
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QuickSelect.temp = null;
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}
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}
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}
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