Python. 3 lines. faster than 100.00%. cool & clear solution.
\tclass Solution:\n\t\tdef getSmallestString(self, n: int, k: int) -> str:\n\t\t\tz = (k - n) // 25\n\t\t\tunique = chr(k - z * 25 - n + 97) if n - z else ""\n\
\tclass Solution:\n\t\tdef getSmallestString(self, n: int, k: int) -> str:\n\t\t\tz = (k - n) // 25\n\t\t\tunique = chr(k - z * 25 - n + 97) if n - z else ""\n\t\t\treturn "a"*(n-z-1) + unique + "z"*z
['Python', 'Python3']