快速排序

快速排序由C. A. R. Hoare在1962年提出。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，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，然后再按此方法对这两部分数据分别进行快速排序，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。

实现代码：

package com.charles.algorithm;

public class QuickSort {

    public static void main(String[] args) {

        int[] data = new int[] { 4, 6, 8, 2, 5, 1, 0, 4, 8, 2 };
        quickSort3(data, 0, data.length - 1);
        for (int item : data) {
            System.out.print(item + " ");
        }
        System.out.println();
    }

    // One: make the first item to be index
    public static void quickSort(int[] data, int low, int high) {
        if (low >= high) {
            return;
        }
        int clow = low, chigh = high;
        int index = data[low];
        while (low < high) {
            while (low < high && data[high] >= index) {
                high--;
            }
            data[low] = data[high];
            while (low < high && data[low] <= index) {
                low++;
            }
            data[high] = data[low];
        }
        data[low] = index;
        quickSort(data, low + 1, chigh);
        quickSort(data, clow, high - 1);
    }

    // Two: make the middle item to be index
    public static void quickSort2(int[] data, int low, int high) {
        if (low >= high) {
            return;
        }

        int clow = low, chigh = high;
        int middle = (low + high) / 2;
        int index = data[middle];
        while (low <= high) {
            while (low <= high && data[low] < index) {
                low++;
            }
            while (low <= high && data[high] > index) {
                high--;
            }
            if (low < high) {
                middle = data[low];
                data[low++] = data[high];
                data[high--] = middle;
            } else {
                low++;
                high--;
            }

        }
        quickSort2(data, clow, high);
        quickSort2(data, low, chigh);
    }

    // Three: make the middle item to be index
    public static void quickSort3(int[] data, int low, int high) {
        if (low >= high) {
            return;
        }

        int clow = low, chigh = high;
        int middle = (low + high) / 2;
        int index = data[middle];
        while (low < high) {
            while (low < high && data[low] < index) {
                low++;
            }
            while (low < high && data[high] > index) {
                high--;
            }
            if (low < high) {
                middle = data[low];
                data[low++] = data[high];
                data[high--] = middle;
            } else {
                if (data[low] > index) {
                    high--;
                } else {
                    low++;
                }
                break;
            }

        }
        quickSort3(data, clow, high);
        quickSort3(data, low, chigh);
    }

}