《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(c) for c in stored_characters)
    digits = tuple(ord(c) 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)