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Approach\n Describe your approach to solving the problem. \nThe solution uses a recursive approach with backtracking. For each string in the array, it explores 
# Approach\n<!-- Describe your approach to solving the problem. -->\nThe solution uses a recursive approach with backtracking. For each string in the array, it explores two choices: whether to include the current string in the concatenation or not. To ensure uniqueness, the code keeps track of the frequency of characters encountered so far using the **freq** array.\n\nThe **f** function is a recursive function that takes the current index (**i**), the frequency array (**freq**), and the array of strings (**arr**). It calculates the maximum length recursively by considering both choices: taking or not taking the current string. The **take** variable stores the length when the current string is taken, and **noTake** stores the length when the current string is not taken. The base case is when all strings are processed, and the function returns 0.\n\n# Code\n```\nclass Solution {\npublic:\n    // Helper function for recursive calculation\n    int f(int i , vector<int> &freq , vector<string> &arr){\n\n        // Base case: If we have processed all strings in the array, return 0\n        if(i == arr.size()) return 0;\n\n        // Initialize variables for the two choices: take or don\'t take the current string\n        int take = 0 , noTake = 0;\n\n        // Recursively calculate the maximum length without taking the current string\n        noTake = f(i+1 , freq , arr);\n\n        // Check if the current string can be taken by ensuring unique characters\n        bool canBeTaken = true;\n        for(int j=0 ; j<arr[i].size() ; j++){\n            char cur = arr[i][j];\n            int idx = cur - 97;\n\n            // If the character has been encountered before, set canBeTaken to false\n            if(freq[idx] != 0) canBeTaken = false;\n\n            // Mark the character as encountered\n            freq[idx]++;\n        }\n\n        // If the current string can be taken, calculate the length and recurse\n        if(canBeTaken){\n            take = arr[i].size() + f(i+1 , freq , arr);\n        }\n\n        // Undo the changes to the frequency array for backtracking\n        for(int j=0 ; j<arr[i].size() ; j++){\n            char cur = arr[i][j];\n            int idx = cur - 97;\n            freq[idx]--;\n        }\n\n        // Return the maximum length between taking and not taking the current string\n        return max(take , noTake);\n    }\n\n    int maxLength(vector<string>& arr) {\n        // Initialize frequency array for tracking character occurrences\n        vector<int> freq(26 , 0);\n\n        // Call the recursive function starting from the first string\n        return f(0 , freq , arr);\n    }\n};\n\n```
['Backtracking', 'C++']