transform function
Implementation
void transform(List<double> real, List<double> imag) {
var n = real.length;
if (n != imag.length) throw new RangeError("实部虚部长度不匹配");
if (n == 0)
return;
else if ((n & (n - 1)) == 0)
transformRadix2(real, imag);
else
transformBluestein(real, imag);
}