1576. 替换所有的问号

from typing import List


class Solution:
    def modifyString(self, s: str) -> str:
        s = list(s)

        def find_char_not_in(ss: List[str]) -> str:
            candidates = set('abcdefghijklmnopqrstuvwxyz').difference(ss)
            return candidates.pop()

        for i, char in enumerate(s):
            if char == '?':
                s[i] = find_char_not_in(s[i - 1: i + 2])

        return ''.join(s)