Letter Combinations of a Phone Number

Description

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.

Example:

Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:

Although the above answer is in lexicographical order, your answer could be in any order you want.

Solution(javascript)

/**
 * @param {string} digits
 * @return {string[]}
 */
let letterCombinations = function(digits) {
    const map = {
        "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"]
    };
    
    function appendLetter(pee, per) {
        let a = [];
        for(let i = 0; i < pee.length; i++){
            for(let j = 0; j < per.length; j++){
                a.push(pee[i] + per[j]);
            }    
        }
        return a;
    }
    
    function combLetter(arr, nums) {
        if(!nums) {
            return arr;
        }
        return combLetter(appendLetter(arr, map[nums[0]]), nums.slice(1));
    }
    
    if(!digits)
        return [];
    return combLetter(map[digits[0]], digits.slice(1));
};