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;
     }