"""
Exercise 1: Letter Combinations of a Phone Number (LeetCode 17)

Given digits 2-9, return every letter combination (2="abc" ... 9="wxyz").
Empty input -> []. Backtrack one digit at a time.

  letter_combinations("23") -> ["ad","ae","af","bd","be","bf","cd","ce","cf"]
"""

def letter_combinations(digits):
    # TODO
    return []


# --- self-check: do not modify below this line ---
a = letter_combinations("23")
print(a, letter_combinations(""), letter_combinations("7"))
ok = sorted(a) == ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"] and letter_combinations("") == [] and len(letter_combinations("7")) == 4
print("PASS" if ok else "FAIL")