# 构造字典，'A' --> 0 ...
def char_2_num(x):
  list_s = []
  list_num = []
  for i in range(26):
    c = chr(i + 65)
    list_s.append(c)
    list_num.append(i)
  c_2_n = dict(map(lambda x, y: [x, y], list_s, list_num))
  return c_2_n[f'{x}']


# 构造字典，0 ---> 'A' ...
def num_2_char(x):
  list_s = []
  list_num = []
  for i in range(26):
    c = chr(i + 65)
    list_s.append(c)
    list_num.append(i)
  n_2_c = dict(map(lambda x, y: [x, y], list_num, list_s))
  print(n_2_c[x],end='')

# 编码
def encode():
  s = input('输入需要编码的字符： ')
  print('编码后的结果为： ',end='')
  for j in s:
    if j.isspace():
      print(' ',end='')
    else:
      ek = a * char_2_num(j) + b
      result = ek % 26
      num_2_char(result)

# 求模26下a的逆
def inv_(x):
  for inv_a in range(1,26,2):
    for j in range(27):
      if x * inv_a == 26 * j + 1:
        return inv_a

# 解码
def decode():
  s = input('输入需要解码的字符： ')
  print('解码后的结果为： ',end='')
  for j in s:
    if j.isspace():
      print(' ',end='')
    else:
      dk = inv_(a) * (char_2_num(j) - b)
      result = dk % 26
      num_2_char(result)

# 输入指令
answer = input(f'请输入所需的操作：编码/E or 解码/D: ')

# 输入参数a，b
a = int(input('请输入a：'))
b = int(input('请输入b: '))

try:
  if answer.upper() == 'E':
    encode()
  elif answer.upper() =='D':
    decode()
  else:
    print('输入错误！')
except KeyError:
  print('请正确输入大写字母！')