快速排序

public class QuickSort {

    public static void main(String[] args) {

        int[] arr1 = {};
        int[] arr2 = {1,2,34,5,6,2,3445,3,4};
        int[] arr3 = {2222,1};
        quickSort(arr1);
        quickSort(arr2);
        quickSort(arr3);
        printArr(arr1);
        printArr(arr2);
        printArr(arr3);
    }

    /**
     * 快排
     * @param arr
     */
    public static void quickSort (int[] arr) {
        help(arr, 0, arr.length -1 );
    }

    public static void help (int[] arr, int left, int right) {

        int head = left;
        int tail = right;

        if (tail <= head) {
             return;
        }

        // 标准值
        int standard = arr[head];
        int standardIndex = head;
        head ++;

        // 快排
        while(head <= tail) {
            // 先从右往左
            // 大于的话 tail 就一直往左移
            while(tail >= head && arr[tail] >= standard) {
                tail--;
            }
            if (tail < head) {
                break;
            }
            // 交换
            arr[standardIndex] = arr[tail];
            standardIndex = tail;

            // 从左往右移
            // 小于的话，head 就一直往右移
            while (tail >= head && arr[head] < standard) {
                head ++;
            }
            if (tail < head) {
                break;
            }
            // 交换
            arr[standardIndex] = arr[head];
            standardIndex = head;
        }

        // 归位
        arr[standardIndex] = standard;

        // 递归
        help(arr, left, standardIndex - 1);
        help(arr, standardIndex + 1, right);
    }

    public static void printArr(int[] arr) {
        System.out.println("=================");
        for (int i = 0; i < arr.length; i++) {
            System.out.println("arr["+ i + "] = " + arr[i]);
        }
        System.out.println("=================");
        System.out.println();
    }
}