思路来源
FFT详解
https://blog.csdn.net/ggn_2015/article/details/68922404
FFT代码
https://blog.csdn.net/hlyfalsy/article/details/9938547
https://blog.csdn.net/XieNaoban/article/details/69486299
FFT+NTT的理解
https://blog.csdn.net/qq_37136305/article/details/81184873
关于蝴蝶操作
https://blog.csdn.net/JuneWindy/article/details/79156335
关于IDFT矩阵的逆的证明
https://blog.csdn.net/f_zyj/article/details/76037583
FFT例题 3idiots
http://www.cnblogs.com/kuangbin/archive/2013/07/24/3210565.html
牛客十一赛例题 FFT+原根
https://blog.csdn.net/u013534123/article/details/82919834
感谢这么多答主,让我用了10h终于理解了FFT。