Create a function that returns all possible permutations of a given string, ensuring that no permutation is repeated in the result. ### Constraints: - The input string will only consist of lowercase English letters. Its length will not exceed 8 characters. ### Examples: 1. Input: 'a' Output: ['a'] 2. Input: 'aba' Output: ['aab', 'aba', 'baa']

``` def permute_unique(s): results = [] def backtrack(path, used): if len(path) == len(s): results.append(''.join(path)) return for i in range(len(s)): if used[i]: continue if i > 0 and s[i] == s[i - 1] and not used[i - 1]: continue used[i] = True path.append(s[i]) backtrack(path, used) path.pop() used[i] = False s = sorted(s) backtrack([], [False] * len(s)) return results # Example usages: print(permute_unique('a')) # ['a'] print(permute_unique('aba')) # ['aab', 'aba', 'baa'] ```