如何只打印字符串的唯一排列？

def unique_permutations(string):
    result = []
    used = [False] * len(string)
    backtrack(string, [], used, result)
    return result

def backtrack(string, path, used, result):
    if len(path) == len(string):
        result.append(''.join(path))
        return

    for i in range(len(string)):
        if used[i]:
            continue
        if i > 0 and string[i] == string[i-1] and not used[i-1]:
            continue
        used[i] = True
        path.append(string[i])
        backtrack(string, path, used, result)
        used[i] = False
        path.pop()

# 示例用法
string = "aab"
permutations = unique_permutations(string)
for permutation in permutations:
    print(permutation)