phone number letters

phoneletters/phoneletters.go

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+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)
+}

phoneletters/phoneletters_test.go

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+package phoneletters_test
+
+import (
+	"testing"
+
+	"git.jxs.me/leetgo/phoneletters"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEm(t *testing.T) {
+	require.Equal(t, []string{}, phoneletters.Combinations(""))
+}
+
+func TestSingle(t *testing.T) {
+	require.Equal(t, []string{"a", "b", "c"}, phoneletters.Combinations("2"))
+}
+
+func TestDoble(t *testing.T) {
+	expected := []string{
+		"ad", "ae", "af",
+		"bd", "be", "bf",
+		"cd", "ce", "cf",
+	}
+
+	require.Equal(t, expected, phoneletters.Combinations("23"))
+}

phoneletters/readme.md

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+17. Letter Combinations of a Phone Number
+Medium
+
+Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
+
+A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
+
+Example 1:
+
+Input: digits = "23"
+Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
+
+Example 2:
+
+Input: digits = ""
+Output: []
+
+Example 3:
+
+Input: digits = "2"
+Output: ["a","b","c"]
+
+ 
+
+Constraints:
+
+    0 <= digits.length <= 4
+    digits[i] is a digit in the range ['2', '9'].