分治法-快速排序

算法思想：分治法
實際問題：快速排序
編寫語言：Java

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Java代碼

//本篇博文代碼主要有兩種基准選擇方式：基准=低下標處的值，基准=隨機值
import java.util.Random;

public class QuickSort
{
    public static void main(String[] args)
    {
        int[] ary = new int[] {1, 3, 4, 5, 2, 7, 0, 6, 9, 8};

        System.out.print("排序前的數組：");
        for(int i = 0; i < ary.length; i++)
            System.out.print(ary[i] + " ");
        System.out.println();

        sort(ary);

        System.out.print("排序后的數組：");
        for(int i = 0; i < ary.length; i++)
            System.out.print(ary[i] + " ");
        System.out.println();
    }

    public static void sort(int[] a)
    {
        sort(a, 0, a.length - 1);
    }

    public static void sort(int[] a, int low, int high)
    {
        //當low == high時就返回
        //確保數組元素為1時就停止划分，防止數組下標越界
        if(low < high)
        {
            int pivot = randPart(a, low, high);
            sort(a, low, pivot - 1);
            sort(a, pivot + 1, high);
        }
    }

    //划分尋找基准
    public static int part(int[] a, int low, int high)
    {
        int pivot = a[low];

        while(low < high)
        {
            //1、從右往左找比基准小的數
            while(low < high && a[high] > pivot)
                high--;
            //a處賦值
            if(low < high)
                a[low] = a[high]; //此時是 a[high] < pivot, a[low] < pivot
            //2、從左往右找比基准大的數
            while(low < high && a[low] <= pivot)
                low++;
            //3、一次尋找結束，交換兩個值
            //b處賦值
            if(low < high)
                a[high] = a[low]; //此時是 a[high] > pivot, a[low] < pivot
            //a、b兩處賦值，相當於一次交換，只是分開了
        }

        //將pivot放到left和right相遇的地方
        a[high] = pivot;

        return high;
    }

    //划分尋找基准-隨機化優化
    public static int randPart(int[] a, int low, int high)
    {
        Random r = new Random();
        //隨機產生一個 low 到 high 的整數
        int flag = low + r.nextInt(high - low + 1);

        int pivot = a[flag];

        //此處交換保證 1 處的賦值不出錯，
        //因為只要原 a[low] < pivot，那么這個交換算法就失敗了
        a[flag] = a[low];
        a[low] = pivot;


        while(low < high)
        {
            //1、從右往左找比基准小的數
            while(low < high && a[high] > pivot)
                high--;
            if(low < high)
                a[low] = a[high];
            //2、從左往右找比基准大的數
            while(low < high && a[low] <= pivot)
                low++;
            if(low < high)
                a[high] = a[low];
        }

        //將pivot放到low和high相遇的地方
        a[high] = pivot;

        return high;
    }
}

運行結果