package phoneletters

var digitToLetters = map[rune]string{
	'2': "abc",
	'3': "def",
	'4': "ghi",
	'5': "jkl",
	'6': "mno",
	'7': "pqrs",
	'8': "tuv",
	'9': "wxyz",
}

func letterCombinations(digits string) []string {
	if digits == "" {
		return []string{}
	}

	combos := make([]string, 0, len(digits)*4)
	rest := letterCombinations(digits[1:])
	if len(rest) == 0 {
		rest = []string{""}
	}

	letters := digitToLetters[rune(digits[0])]
	for _, letter := range letters {
		for _, r := range rest {
			combos = append(combos, string(letter)+r)
		}
	}

	return combos
}

func Combinations(digits string) []string {
	return letterCombinations(digits)
}