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);
}