17. Letter Combinations of a Phone Number

Problem:

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.

200px-Telephone-keypad2

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:

ONE

JavaScript specific optimization.

Array.prototype.push accepts arbitrary arguments which enables tighter loops.

Also, appending string is faster than prepending.

///**
 * @param {string} digits
 * @return {string[]}
 */let letterCombinations = function (digits) {if (digits.length <= 0) {return [];}const letters = [,,['a', 'b', 'c'],['d', 'e', 'f'],['g', 'h', 'i'],['j', 'k', 'l'],['m', 'n', 'o'],['p', 'q', 'r', 's'],['t', 'u', 'v'],['w', 'x', 'y', 'z']];let result = [''];for (let i = 0; i < digits.length; i++) {const arr = letters[digits[i]];let newResult = [];
        arr.forEach((c) => newResult.push(...result.map((r) => r + c)));
        result = newResult;}return result;};

TWO

General recursive DFS solution.

///**
 * @param {string} digits
 * @return {string[]}
 */let letterCombinations = function (digits) {const letters = [, , 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz'];const result = [];if (digits.length > 0) {dfs(digits, 0, '', letters, result);}return result;};function dfs(digits, idigit, path, letters, result) {if (idigit >= digits.length) {
        result.push(path);return;}const str = letters[digits[idigit]];for (let i = 0; i < str.length; i++) {dfs(digits, idigit + 1, path + str[i], letters, result);}}

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