Python中《Dive into python3》alphametics.py 代码请教

问题 1：为什么要按照以下顺序组合字母，stored_characters = ''.join(unique_characters)行不行，只是顺序不同

stored_characters = ''.join(first_letters) +  \
    ''.join(unique_characters - first_letters) 

问题 2： 为什么要排除前 n 个元素含有 zero 的排列？

if zero not in guess[:n]: 

代码如下：

import re
import itertools
def solve(puzzle):
words = re.findall(’[A-Z]+’, puzzle.upper())
unique_characters = set(’’.join(words))
assert len(unique_characters) <= 10, ‘Too many letters’
first_letters = {word[0] for word in words}
n = len(first_letters)
stored_characters = ‘’.join(first_letters) +  

‘’.join(unique_characters - first_letters)
characters = tuple(ord© for c in stored_characters)
digits = tuple(ord© for c in ‘0123456789’)
zero = digits[0]
for guess in itertools.permutations(digits, len(characters)):
if zero not in guess[:n]:
equation = puzzle.translate(dict(zip(characters,guess)))
if eval(equation):
return equation
if name == ‘main’:
import sys
for puzzle in sys.argv[1:]:
print(puzzle)
solution = solve(puzzle)
if solution:
print(solution)