快速排序

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快速排序的基本思想是:通过一趟排序将待排记录分割成两个独立的部门,其中一部门记录的关键字均比另一部分记录的关键字要小,则可分别对这两部分记录继续进行排序,已达到整个序列有序的目的.

整个排序最核心的部分就是:如何通过一趟排序将待排记录分割成两个独立的部门,这个功能由partition函数实现:partition函数要做的就是,先选取一个关键字(这个关键字的选取很有讲究,直接关心到快速排序的效率),然后想办法把它放到一个合适的位置,使得它左边的数都比它小,而右边的数都比它大.我们称这样的关键字为枢轴.

partition的两种实现如下:

实现一:

public static int partition(int[] data, int start, int end){int temp = data[start]; // 把第一个数作为基准数int low = start; int high = end;while(low < high){while(low<high && data[high]>=temp){high--;}swap(data, high, low);while(low<high && data[low]<=temp){low++;}swap(data, low, high);}return low;}

实现二:

public static int partition(int[] data, int start, int end){int index = start;int small = index -1;swap(data, index, end);for(int i=start; i<end; i++){if(data[i] <= data[end]){small++;if(small != i){swap(data, small, i);}}}small++;swap(data, small, end);return small;}

两种方法的完整版本如下:

实现一:

package cn.ccnu.sort;public class QuickTwo {public static void quickSort(int[] data, int start, int end){if(data == null){throw new RuntimeException("input error");}if(start < end){int index = partition(data, start, end);quickSort(data, start, index-1);quickSort(data, index+1, end);}}public static int partition(int[] data, int start, int end){int temp = data[start]; // 把第一个数作为基准数int low = start; int high = end;while(low < high){while(low<high && data[high]>=temp){high--;}swap(data, high, low);while(low<high && data[low]<=temp){low++;}swap(data, low, high);}return low;}private static void swap(int[] data, int a, int b) {int temp = data[a];data[a] = data[b];data[b] = temp;}public static void main(String[] args) {int[] data = {10, 3, 12, 5, 4, 9, 5, 7, 6};int start = 0;int end = data.length-1;quickSort(data, start, end);for (int i : data) {System.out.print(i + "  ");}}}

实现二:

package cn.ccnu.sort;public class Quick {public static void quickSort(int[] data, int start, int end){if(data == null){throw new RuntimeException("input error");}if(start < end){int index = partition(data, start, end);quickSort(data, start, index-1);quickSort(data, index+1, end);}}public static int partition(int[] data, int start, int end){int index = start;int small = index -1;swap(data, index, end);for(int i=start; i<end; i++){if(data[i] <= data[end]){small++;if(small != i){swap(data, small, i);}}}small++;swap(data, small, end);return small;}private static void swap(int[] data, int a, int b){int temp = data[a];data[a] = data[b];data[b] = temp;}public static void main(String[] args) {int[] data = {10, 3, 12, 5, 4, 9, 4, 7, 4};int start = 0;int end = data.length-1;quickSort(data, start, end);for (int i : data) {System.out.print(i + "  ");}}}

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