def unique_in_order(inp):
    if len(inp) == 0:
        return []
    
    res = [inp[0]]
    for item in inp:
        if item != res[-1]:
            res.append(item)

    return res


samples = [
    "",
    "a",
    "aa",
    "aaa",
    "ab",
    "aba",
    "aab",
    "aabb",
    "aabba",
]

for sample in samples:
    print(sample, unique_in_order(sample))