中文说明：
本程序是用于处理端点效应时的信号延拓方法，次程序是用于前段延拓，对于后端延拓的话直接把原信号进行翻转后再进行延拓，延拓后将信号再翻转回来就行。
English Description：
This program is used to deal with the end effect of the signal extension method, the second program is used for the front extension, for the back extension, then directly flip the original signal and then carry out the extension, and then flip the signal back after the extension