快速乘方

aⁿ = a^n/2*a^n/2   , n为even

aⁿ = a^(n-1)/2*a^(n-1)/2 *a  , n为odd

double powerWithUnsignedExponent(double base, unsigned int exponent)
{
      if(0 == exponent)
          return 1;
      if(1 == exponent)
          return base;
      double result = powerWithUnsignedExponent(base, exponnet >> 1);
          result *= result;
      if(1 == exponent & 0x1)     //二进制末尾为1，一定是奇数
          result *= base;

      return result;
}

借助位运算判断奇偶性：

a & 0x1 == 0;    //even偶数

a & 0x1 != 0;     //odd奇数