fft 快速傅里叶变化，cpp程序 一种新的变化，-let everyone have fft program

fft算法的实现，包括时域抽取和频域抽取。可以帮助学生快速了解fft算法的核心思想。-This software is a implementation of fft algorithm, which includes the time-domain and frequency-domain.Using this software, students can quickly understand the core idea of fft algorithm.