python3 simple solution
\nclass Solution:\n    def modifyString(self, s: str) -> str:\n        if s == \'?\':\n            return \'a\'\n        s_set = set(s)\n        available_chars
```\nclass Solution:\n    def modifyString(self, s: str) -> str:\n        if s == \'?\':\n            return \'a\'\n        s_set = set(s)\n        available_chars = set()\n        for char in s:\n            if char == \'?\':\n                if not available_chars:\n                    available_chars = set(\'abcdefghijklmnopqrstuvwxyz\').difference(s_set)\n                s = s.replace(char, available_chars.pop(), 1)\n        return s\n```
['Python3']