main.go

package main

import "fmt"

var letters = map[rune]string{
	'2': "abc",
	'3': "def",
	'4': "ghi",
	'5': "jkl",
	'6': "mno",
	'7': "pqrs",
	'8': "tuv",
	'9': "wxyz",
	'0': "",
	'1': "",
}

func aux(i int, original string, accumulator string, set []string) []string {
	if len(original) == len(accumulator) {
		set = append(set, accumulator)
		return set
	}

	for _, l := range letters[rune(original[i])] {
		current := accumulator + string(l)
		set = aux(i+1, original, current, set)
		current = current[:len(current)-1]
	}
	return set
}

func generateCombinations(s string) []string {
	return aux(0, s, "", []string{})
}

func main() {
	fmt.Println(generateCombinations("234"))
}