面试题11：数值的整数次方

要注意的问题：

1. 指数为0或者是负数

2. 判断小数误差切记不能用 == ，因为计算机内表示小数时（包括float和double型小数）都有误差。判断两个小数是否相等，只能判断它们之差的绝对值是否在一个很小的范围内

bool g_InvalidInput = false;
bool equal(double num1, double num2);
double PowerWithUnsignedExponent(double base, unsigned int exponent);
 
double Power(double base, int exponent)
{
    g_InvalidInput = false;
 
    if(equal(base, 0.0) && exponent < 0)
    {
        g_InvalidInput = true;
        return 0.0;
    }
 
    unsigned int absExponent = (unsigned int)(exponent);
    if(exponent < 0)
        absExponent = (unsigned int)(-exponent);
 
    double result = PowerWithUnsignedExponent(base, absExponent);
    if(exponent < 0)
        result = 1.0 / result;
 
    return result;
}
 

double PowerWithUnsignedExponent(double base, unsigned int exponent)
{
    double result = 1.0;
    /
    for(int i = 1; i <= exponent; ++i)
        result *= base;
 
    return result;
}

bool equal(double num1, double num2)
{
    if((num1 - num2 > -0.0000001)
        && (num1 - num2 < 0.0000001))
        return true;
    else
        return false;
}