176 lines
6.2 KiB
Java
176 lines
6.2 KiB
Java
/*
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* The MIT License (MIT)
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*
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* Copyright © 2020 xrv <xrg@live.com>
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package io.github.ehlxr.sort;
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import java.util.Arrays;
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/**
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* 快速排序
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* <p>
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* 快速排序(Quicksort)是对冒泡排序的一种改进,借用了分治的思想,
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* 它的基本思想是:通过一趟排序将要排序的数据分割成独立的两部分,
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* 其中一部分的所有数据都比另外一部分的所有数据都要小,
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* 然后再按此方法对这两部分数据分别进行快速排序,
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* 整个排序过程可以递归进行,以此达到整个数据变成有序序列。
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*
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* @author ehlxr
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* @since 2020-10-01 16:40.
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*/
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public class QuickSort {
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public static void main(String[] args) {
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// int[] arr = {4, 9, 1, 8, 6, 2};
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int[] arr = new int[800_000];
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for (int i = 0; i < 800_000; i++) {
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arr[i] = (int) (Math.random() * 80_000_000); //生成一个[0, 8000000) 数
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}
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long startTime = System.currentTimeMillis();
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sort(arr, 0, arr.length - 1);
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System.out.printf("排序花费时间 %dms.", System.currentTimeMillis() - startTime);
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}
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/**
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* 快速排序使用分治策略来把一个序列(list)分为两个子序列(sub-lists)。
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* 步骤为:
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* ①. 从数列中挑出一个元素,称为 "基准"(pivot)。
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* ②. 重新排序数列,所有比基准值小的元素摆放在基准前面,所有比基准值大的元素摆在基准后面(相同的数可以到任一边)。
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* 在这个分区结束之后,该基准就处于数列的中间位置。这个称为分区(partition)操作。
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* ③. 递归地(recursively)把小于基准值元素的子数列和大于基准值元素的子数列排序。
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* <p>
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* 递归到最底部时,数列的大小是零或一,也就是已经排序好了。
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* 这个算法一定会结束,因为在每次的迭代(iteration)中,它至少会把一个元素摆到它最后的位置去。
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*
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* @param arr 待排序数组
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* @param l 左边界
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* @param r 右边界
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*/
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public static void sort(int[] arr, int l, int r) {
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if (l >= r) {
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return;
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}
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if (arr == null || arr.length <= 0) {
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return;
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}
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int i = l, j = r,
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// 一般选择数组左边界作为基准
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k = l;
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while (l < r) {
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// 首先循环递减右边界,直到找到小于基准的元素,相互交换
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while (l < r && arr[r] >= arr[k]) {
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r--;
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}
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swap(arr, k, r);
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k = r;
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// 其次循环递增左边界,直到找到大于基准的元素,相互交换
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while (l < r && arr[l] <= arr[k]) {
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l++;
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}
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swap(arr, k, l);
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k = l;
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// 循环以上步骤,直到 l 和 r 相遇
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}
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// arr[k] = p;
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// System.out.println("Sorting: " + Arrays.toString(arr));
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sort(arr, i, k - 1);
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sort(arr, k + 1, j);
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}
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/*
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* 左右指针法
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*/
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// public static void sort(int[] arr, int low, int high) {
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// if (arr == null || arr.length <= 0) {
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// return;
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// }
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// if (low >= high) {
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// return;
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// }
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//
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// int left = low;
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// int right = high;
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//
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// int key = arr[left];
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//
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// while (left < right) {
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// while (left < right && arr[right] >= key) {
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// right--;
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// }
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// while (left < right && arr[left] <= key) {
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// left++;
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// }
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// if (left < right) {
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// swap(arr, left, right);
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// }
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// }
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// swap(arr, low, left);
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// System.out.println("Sorting: " + Arrays.toString(arr));
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// sort(arr, low, left - 1);
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// sort(arr, left + 1, high);
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// }
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/*
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* 挖坑法递归
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*/
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// public static void sort2(int[] arr, int low, int high) {
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// if (arr == null || arr.length <= 0) {
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// return;
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// }
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// if (low >= high) {
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// return;
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// }
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//
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// int left = low;
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// int right = high;
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// // 挖坑1:保存基准的值
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// int temp = arr[left];
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//
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// while (left < right) {
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// while (left < right && arr[right] >= temp) {
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// right--;
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// }
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// //坑2:从后向前找到比基准小的元素,插入到基准位置坑1中
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// arr[left] = arr[right];
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// while (left < right && arr[left] <= temp) {
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// left++;
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// }
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// //坑3:从前往后找到比基准大的元素,放到刚才挖的坑2中
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// arr[right] = arr[left];
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// }
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// //基准值填补到坑3中,准备分治递归快排
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// arr[left] = temp;
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// System.out.println("Sorting: " + Arrays.toString(arr));
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// sort2(arr, low, left - 1);
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// sort2(arr, left + 1, high);
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// }
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public static void swap(int[] arr, int low, int high) {
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int tmp = arr[low];
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arr[low] = arr[high];
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arr[high] = tmp;
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}
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} |