Dart Solution
\nclass Solution {\n  String modifyString(String s) {\n    final letters = List.generate(26, (i) => String.fromCharCode(i + 97));\n    int index = s.indexOf(\'?
```\nclass Solution {\n  String modifyString(String s) {\n    final letters = List.generate(26, (i) => String.fromCharCode(i + 97));\n    int index = s.indexOf(\'?\');\n    while (index != -1) {\n      String first = \'\';\n      String second = \'\';\n      if (index > 0) first = s[index - 1];\n      if (index < s.length - 1) second = s[index + 1];\n\n      for (int i = 0; i < letters.length; i++) {\n        if (first != letters[i] && second != letters[i]) {\n          s = s.replaceRange(index, index + 1, letters[i]);\n          break;\n        }\n      }\n      index = s.indexOf(\'?\');\n    }\n    return s;\n  }\n}\n```
['Dart']