Implement a function that takes a string as input and returns all unique permutations of the string sorted in lexicographical order. ### Constraints: - The input string will contain lowercase letters only and will have at most length 9. ### Examples: 1. Input: 'abb' Output: ['abb', 'bab', 'bba'] 2. Input: 'a' Output: ['a']

``` def unique_permutations(s): from collections import Counter res = [] def backtrack(path, counter): if len(path) == len(s): res.append(''.join(path)) return for char in counter: if counter[char] > 0: path.append(char) counter[char] -= 1 backtrack(path, counter) path.pop() counter[char] += 1 backtrack([], Counter(s)) return sorted(res) # Example usages: print(unique_permutations('abb')) # ['abb', 'bab', 'bba'] print(unique_permutations('a')) # ['a'] ```