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Letter Combinations of a Phone Number

Problem statement​

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 digit 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'].

My solution​

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
    const mappings = new Map([
        ["0", new Set()],
        ["1", new Set()],
        ["2", new Set(["a", "b", "c"])],
        ["3", new Set(["d", "e", "f"])],
        ["4", new Set(["g", "h", "i"])],
        ["5", new Set(["j", "k", "l"])],
        ["6", new Set(["m", "n", "o"])],
        ["7", new Set(["p", "q", "r", "s"])],
        ["8", new Set(["t", "u", "v"])],
        ["9", new Set(["w", "x", "y", "z"])],
    ]);
    const results = [];
    function walk(index, prefix) {
        if (index >= digits.length) {
            return results.push(prefix);
        }
        const digit = digits.charAt(index);
        const nums = mappings.get(digit)
        
        for (const char of nums.keys()) {
            walk(index + 1, prefix + char)
        }
    }
    
    if (digits.length > 0) {
        walk(0, "")
    }
    return results;
};