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原题是这样的：A Caesar cipher is a simple substitution cipher based on the idea of shifting each letter of the plaintext

message a fixed number (called the key) of positions in the alphabet. For example, if the key value is

2, the word “Sourpuss” would be encoded as “Uqwtrwuu.” The original message can be recovered by

“reencoding” it using the negative of the key.

Write a program that can encode and decode Caesar ciphers. The input to the program will be a string

of plaintext and the value of the key. The output will be an encoded message where each character in

the original message is replaced by shifting it key characters in the ASCII character set. For example,

if ch is a character in the string and key is the amount to shift, then the character that replaces ch can

be calculated as: chr(ord(ch) + key).

然后我们老师还要求可以把key取任意值，然后像x\y\z这些排在字母表后面的，可以在往后推几位时，回到a\b\c这些排在前面的。然后就是能把小写字母变成大写的，谢谢啦！