Maximum length of a concatenated string with unique characters

We will use a set to filter out the strings with duplicate characters. Then we iterate over each sting and check if it has duplicate characters. If not, we will add it to the set. Then we will iterate over all the strings in the set and check if the string can be concatenated with the current string. If yes, we will update the maximum length.

class Solution:
    def maxLength(self, arr: List[str]) -> int:
        maxlen = 0
        unique = ['']

        def isvalid(s):
            return len(s) == len(set(s))

        for word in arr:
            for j in unique:
                tmp = word + j
                if isvalid(tmp):
                    unique.append(tmp)
                    maxlen = max(maxlen, len(tmp))

        return maxlen

Time Complexity: O(n^2)
Space Complexity: O(n)