unsigned int reverse(unsigned int num, int bits)
{
int ii;
unsigned int n = num&1;
for(ii = 1; ii < bits; ii++)
{
num >>= 1;
n <<= 1;
n |= num&1;
}
return n;
}

用个表效率是最高的了。如用个256格的表，则每八个比特，只要一次运算。理论上效率提高了八倍。