给一个长度为 的字符数组和一个长度为k的01数组req，根据要求编写折叠加密和解密的算法。
折叠加密方法举例：
reg[i]=0,表示从左到右折叠
a b c d e f g h  折叠加密后      d c b a
                             e f g h
d c b a        折叠加密后       f e
e f g h                         c d
                                b a
                                g h
reg[i]=1,表示从右到左折叠
a b c d e f g h  折叠加密后        h g f e
                                   a b c d
f e           折叠加密后       h
c d                            a
b a                            d
g h                            e
                               f
                               c
                               b
                               g