说明：  无限长的序列也往往可以用有限长序列来逼近。对于有限长的序列我们可以使用离散傅立叶变换（DFT）。DFT是对序列傅立叶变换的等距采样。是序列傅立叶变换的等距采样。 本源码是对DCT、DFT、FFT的matlab实现
(Infinitely long sequence is often a long sequence with limited approximation. For the finite length of the sequence we can use the discrete Fourier transform (DFT). DFT is a sequence of equidistant sampling Fourier transform. Is a sequence of equidistant sampling Fourier transform. The source is the DCT, DFT, FFT in matlab implementation)