How to Find the Unique String in Python

The challenge

There is an array of strings. All strings contain similar letters except one. Try to find it!

find_uniq([ 'Aa', 'aaa', 'aaaaa', 'BbBb', 'Aaaa', 'AaAaAa', 'a' ]) # => 'BbBb'
find_uniq([ 'abc', 'acb', 'bac', 'foo', 'bca', 'cab', 'cba' ]) # => 'foo'

Strings may contain spaces. Spaces are not significant, only non-spaces symbols matter.
E.g. A string that contains only spaces is like an empty string.

It’s guaranteed that the array contains more than 3 strings.

The solution in Python code

Option 1:

def find_uniq(arr):
    arr.sort(key=lambda x: x.lower())
    arr1 = [set(i.lower()) for i in arr]
    return arr[0] if arr1.count(arr1[0]) == 1 and str(arr1[0]) != 'set()' else arr[-1]

Option 2:

def find_uniq(arr):
    for word in set(arr):
        for letter in set(word):
            if sum([1 if letter in w else 0 for w in arr]) == 1:
                return word
            else: continue

Option 3:

from collections import Counter
def find_uniq(arr):
    res = Counter(''.join(arr)).most_common()
    return ''.join([x for x in arr if res[-1][0] in x])

Test cases to validate our solution

test.describe('should handle sample cases')
test.assert_equals(find_uniq([ 'Aa', 'aaa', 'aaaaa', 'BbBb', 'Aaaa', 'AaAaAa', 'a' ]), 'BbBb')
test.assert_equals(find_uniq([ 'abc', 'acb', 'bac', 'foo', 'bca', 'cab', 'cba' ]), 'foo')
test.assert_equals(find_uniq([ '    ', 'a', '  ' ]), 'a')