Phone Letter Combinations

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Examples

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

letter_combinations("532") ➞ ["jda", "jdb", "jdc", "jea", "jeb", "jec", "jfa", "jfb", "jfc", "kda", "kdb", "kdc", "kea", "keb", "kec", "kfa", "kfb", "kfc", "lda", "ldb", "ldc", "lea", "leb", "lec", "lfa", "lfb", "lfc"]

Notes

N/A