diff --git a/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number.go b/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number.go
index d03c853d3..fa59a8637 100644
--- a/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number.go
+++ b/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number.go
@@ -1 +1,33 @@
 package letter_combinations_of_a_phone_number
+
+func letterCombinations(digits string) []string {
+	if digits == "" {
+		return nil
+	}
+	m := map[rune][]string{
+		'2': {"a", "b", "c"},
+		'3': {"d", "e", "f"},
+		'4': {"g", "h", "i"},
+		'5': {"j", "k", "l"},
+		'6': {"m", "n", "o"},
+		'7': {"p", "q", "r", "s"},
+		'8': {"t", "u", "v"},
+		'9': {"w", "x", "y", "z"},
+	}
+	ans := make([]string, 1)
+	for _, k := range digits {
+		idx, t := 0, make([]string, len(ans))
+		copy(t, ans)
+		for _, s := range t {
+			for _, c := range m[k] {
+				if idx < len(ans) {
+					ans[idx] = s + c
+				} else {
+					ans = append(ans, s+c)
+				}
+				idx++
+			}
+		}
+	}
+	return ans
+}
diff --git a/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number_test.go b/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number_test.go
index d03c853d3..346b1eaa3 100644
--- a/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number_test.go
+++ b/problems/letter-combinations-of-a-phone-number/letter_combinations_of_a_phone_number_test.go
@@ -1 +1,30 @@
 package letter_combinations_of_a_phone_number
+
+import (
+	"reflect"
+	"testing"
+)
+
+type caseType struct {
+	input    string
+	expected []string
+}
+
+func TestLetterCombinations(t *testing.T) {
+	tests := [...]caseType{
+		{
+			input:    "",
+			expected: nil,
+		},
+		{
+			input:    "23",
+			expected: []string{"ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"},
+		},
+	}
+	for _, tc := range tests {
+		output := letterCombinations(tc.input)
+		if !reflect.DeepEqual(output, tc.expected) {
+			t.Fatalf("input: %v, output: %v, expected: %v", tc.input, output, tc.expected)
+		}
+	}
+}