给定一个数，求出不大于该数的最大的2的幂，例：17->16,256->256;

C# codes as below:

 

        static int GetNum(int value)
        {
            if (value == 0)
                return 0;

            int max=32;
            int min = 0;
            int actual=0;

            int mark = 1;

            if (value < 0)
            {
                mark = -1;
                value*=-1;
            }

            while (max > min)
            {
                actual = (max + min) / 2;

                if (value < 1 << actual)
                {
                    max = actual;
                }
                else
                {
                    if (max - min == 1)
                        break;

                    min = actual;
                }
            }

            return mark * 1 << ((max + min) / 2);
        }