## 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#

``````def maxLength(arr: List[str]) -> int:
result = [float('-inf')]

unique_char("", arr, 0, result)

if not result[0] == float('-inf'):
return result[0]
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[0] = max(result[0],len(cur + arr[index]))
# Make a new call with concatenate words
unique_char(cur + arr[index], arr, index + 1,result)
``````