快速排序

3 月，跳不动了？>>>

1、思想

    每次选出一个基准，然后将数组剩下的元素分为两部分，一部分小于基准放到左边，一部分大于基准放到右边。然后对基准左右两部分分别做同样处理，分治思想。

2、时间复杂度

    最快 O(nlogn) ，最坏 O (n^2) ，平均 O(nlogn) 。

3、代码实现

//常规版
public class quickSort {
    public static void QuickSort(int[] arr){
        QSort(arr, 0, arr.length-1);
    }

    private static void QSort(int[] arr, int low, int hight) {
        int pivot;
        if(low < hight){
            pivot = Partition(arr, low, hight);
            QSort(arr, low, pivot-1);
            QSort(arr, pivot+1, hight);
        }
    }

    private static int Partition(int[] arr, int low, int hight) {
        if(arr == null || low < 0 || hight >= arr.length){
            new Exception();
        }
        int pivotkey = arr[low];
        while(low < hight){
            while(low < hight && arr[hight] >= pivotkey) hight--;
            Swap(arr, low, hight);
            while (low < hight && arr[low] <= pivotkey) low++;
            Swap(arr, low, hight);
        }
        return low;
    }

    private static void Swap(int[] arr, int low, int hight) {
        int temp = arr[low];
        arr[low] = arr[hight];
        arr[hight] = temp;
    }

    public static void main(String[] args) {
        int[] arr = {50,10,90,30,70,40,80,60,20};
        QuickSort(arr);
        for (int array : arr) {
            System.out.print(array+" ");
        }
        System.out.println();
    }
}

/**
 * 三数取中优化版
 */

public class quickSort_up {
    public static void QuickSort(int arr[]){
        if(arr == null || arr.length == 0) return;
        QSort(arr, 0, arr.length-1);
    }

    private static void QSort(int[] arr, int low, int high) {
        int pivot;
        if(low < high){
            pivot = Partition(arr, low, high);
            QSort(arr, low, pivot-1);
            QSort(arr, pivot+1, high);
        }
    }

    private static int Partition(int[] arr, int low, int high) {
        if(arr == null || low < 0 || high >= arr.length){
            new Exception();
        }
        int pivotkey;
        ChoosePivotkey(arr, low, high);
        pivotkey = arr[low];
        while(low < high){
            while(low < high && arr[high] >= pivotkey) high--;
            Swap(arr, low, high);
            while(low < high && arr[low] <= pivotkey) low++;
            Swap(arr, low, high);
        }
        return low;
    }

    //三数取中选择枢轴 将枢轴调到第一个位置
    private static void ChoosePivotkey(int[] arr, int low, int high) {
        int mid = (high + low)/2;
        if(arr[low] > arr[high]){
            //保证左端较小
            Swap(arr, low, high);
        }
        if(arr[mid] > arr[high]){
            //保证中间较小
            Swap(arr, low, high);
        }
        if(arr[mid] > arr[low] ){
            Swap(arr, mid, low);
        }
    }

    private static void Swap(int[] arr, int low, int high) {
        int temp = arr[low];
        arr[low] = arr[high];
        arr[high] = temp;
    }
    public static void main(String[] args) {
        int[] arr = {50,10,90,30,70,40,80,60,20};
        QuickSort(arr);
        for (int array : arr) {
            System.out.print(array+" ");
        }
        System.out.println();
    }

}

/**
 * 此方法为减少无效交换优化法
 */
public class quickSort_up2 {
    public static void QuickSort(int[] arr){
        if(arr == null || arr.length ==0) return;
        QSort(arr, 0, arr.length-1);
    }

    private static void QSort(int[] arr, int low, int high) {
        int pivot;
        if(low < high){
            pivot = Partition(arr, low, high);
            QSort(arr, low, pivot-1);
            QSort(arr, pivot+1,high);
        }
    }

    private static int Partition(int[] arr, int low, int high) {
        if(arr == null || low <0 || high >= arr.length){
            new Exception();
        }
        int pivotkey = arr[low];
        int tempkey = pivotkey;
        while(low < high){
            while (low < high && arr[high] >= pivotkey) high--;
            arr[low] = arr[high];
            while(low < high && arr[low] <= pivotkey) low++;
            arr[high] = arr[low];
        }
        arr[low] = tempkey;
        return low;
    }
    public static void main(String[] args) {
        int[] arr = {50,10,90,30,70,40,80,60,20};
        QuickSort(arr);
        for (int array : arr) {
            System.out.print(array+" ");
        }
        System.out.println();
    }
}

 

来源：oschina

链接：https://my.oschina.net/u/3991724/blog/3207354