## The problem

Given an array of strings `arr`. String `s` is a concatenation of a sub-sequence of `arr` which have unique characters.

Return the maximum possible length of `s`.

## Example test-cases

Example 1:

```Input: arr = ["un","iq","ue"]
Output: 4
Explanation: All possible concatenations are "","un","iq","ue","uniq" and "ique".
Maximum length is 4.
```

Example 2:

```Input: arr = ["cha","r","act","ers"]
Output: 6
Explanation: Possible solutions are "chaers" and "acters".
```

Example 3:

```Input: arr = ["abcdefghijklmnopqrstuvwxyz"]
Output: 26```

## Constraints

• `1 <= arr.length <= 16`
• `1 <= arr[i].length <= 26`
• `arr[i]` contains only lower case English letters.

## How to write the code

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 `````` ``````def maxLength(arr: List[str]) -> int: result = [float('-inf')] unique_char("", arr, 0, result) if not result == float('-inf'): return result return 0 def unique_char(cur, arr, index, result): # End of the array if index == len(arr): return # Iterating from the current word to the end of the array for index in range(index,len(arr)): # If current word + next word have all unique characters if len(set(cur + arr[index])) == len(list(cur + arr[index])): # Compare the actual lenth with the previous max result = max(result,len(cur + arr[index])) # Make a new call with concatenate words unique_char(cur + arr[index], arr, index + 1,result) ``````