Quick Sort(Java)

    public static void main(String[] args)
    {    
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        int[] a = new int[n];
        
        for(int i = 0; i < a.length; i++)
            a[i] = (int)(Math.random() * 100);
        
        System.out.println("Before sort:");
        //System.out.println(Arrays.toString(a));
        long start = System.currentTimeMillis();
        divide(a, 0, a.length - 1);
        long end = System.currentTimeMillis();
        //10000000个测试数据用时约为550ms  相对于归并排序有较大提升
        System.out.println("After sort: ");
        //System.out.println(Arrays.toString(a));
        System.out.println(end - start);
    }
    
    public static void divide(int[] a, int left, int right)
    {
        if(left >= right)   //大于号防止数组为空
            return;
        
        int value = QuickSort(a, left, right);
        //先处理右半边
        divide(a, value + 1, right);   
        //再处理左半边
        divide(a, left, value - 1);   //分割时不能将value作为边界 否则排序数字相同的数组会导致无限递归
    }
    
    public static int QuickSort(int[] a, int left, int right)
    {
        int i = left, j = right + 1;
        int key = a[left];
        
        while(true)
        {    
            while(a[--j] > key)
                if(j == left)   //该判断其实是冗余的 可以去掉
                    break;
            while(a[++i] < key)
                if(i == right) 
                    break;
            if(i >= j)   //不判断相等的情况会导致数组下标越界(传入两个相同的元素) 
                break;
            int temp = a[j];
            a[j] = a[i];
            a[i] = temp;
        }
        
        int t = a[left];
        a[left] = a[j];
        a[j] = t;
        return j;
    }