求一个数的临近的较大的2的整数次幂

unsigned int f2(unsigned int val)
{
    int retval = 1;

    while (retval < val)
    {
        retval <<= 1;
    }

    return retval;
}

unsigned int f1(unsigned int val)
{
    if (val & (val - 1))//至少有两个为1的bit位
    {
        unsigned int leftmost;

        while (val)
        {
            leftmost = val;
            val &= (val - 1);
        }

        return leftmost << 1;
    }
    else
    {
        return val == 0 ? 1 : val;
    }
}

unsigned int f3(unsigned int val){
    {
        if (val & (val - 1)){
            val |= val>>1;
            val |= val>>2;
            val |= val>>4;
            val |= val>>8;
            val |= val>>16;
            return val+1;
        }else{
            return val == 0 ? 1 : val;
        }
    }
}