C++ DFS clean and concise code.
\nclass Solution {\npublic:\n    int maxLength(vector<string>& arr, string str = "", int index = 0) {\n\t    //Use set to check if the string contains all uniqu
```\nclass Solution {\npublic:\n    int maxLength(vector<string>& arr, string str = "", int index = 0) {\n\t    //Use set to check if the string contains all unique characters\n        unordered_set<char>s(str.begin(), str.end());\n        if (s.size() != ((int)str.length())) \n            return 0;\n        \n        int ret = str.length();\n        for (int i = index; i < arr.size(); i++)\n            ret = max(ret, maxLength(arr, str+arr[i], i+1));\n\n        return ret;\n    }\n};\n```
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