第一周：二分法

思想：找一个参照数，将小于该数的放到左边，大于该数的放到右边，分成两部分，然后每部分再不断地递归，直到排好序。

网上的例子：

 class 二分法
    {
        static void Main(string[] args)
        {

           int[] a = { 1, 6, 4, 3, 8, 5, 21, 9, 31, 81, 101, 55, 62, 151, 7, 2, 10 };
            quicksort(a, 0, a.Length - 1);

            for (int i = 0; i < a.Length; i++)
            {
                Console.WriteLine(a[i]);
            }

            Console.Read();
        }

        static void quicksort(int[] a, int start, int end)
        {
            int i;
            if (end - start == 1)
            {
                if (a[start] > a[end])
                {
                    Swap(ref a[start], ref a[end]);
                }

                return;
            }

            if (end - start == 0)
            {
                return;
            }

            i = partition(a, start, end);
            if (i > start)
            {
                quicksort(a, start, i - 1);
            }

            if (i < end)
            {
                quicksort(a, i + 1, end);
            }
        }

        static int partition(int[] a, int start, int end) 
        { 
          int i = start-1; 
          int j = end; 
          int v = a[end]; 
          for (; ; ) 
            { 
                while (i < end && a[++i] < v) ; 
                while (j > start && a[--j] > v) ;             
                if (i >= j) break; 
                Swap(ref a[i], ref a[j]); 
            } 
 
          Swap(ref a[i], ref a[end]); 
          return i; 
        }

        static void Swap(ref int a, ref int b)
        {
            int temp = a;
            a = b;
            b = temp;
        } 
    }