LetterCombination.js

/*
 *
 * Letter Combinations of a Phone Number
 *
 * 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 digits to letters (just like on the telephone buttons) is given below.
 * Note that 1 does not map to any letters.
 * More info: https://leetcode.com/problems/letter-combinations-of-a-phone-number/
 */

/*
 * @param {string} digits
 * @returns {string[]} all the possible combinations
 */

const letterCombinations = (digits) => {
 const length = digits?.length
 const result = []
 if (!length) {
 return result
 }
 const digitMap = {
 2: 'abc',
 3: 'def',
 4: 'ghi',
 5: 'jkl',
 6: 'mno',
 7: 'pqrs',
 8: 'tuv',
 9: 'wxyz'
 }

 const combinations = (index, combination) => {
 let letter
 let letterIndex
 if (index >= length) {
 result.push(combination)
 return
 }
 const digit = digitMap[digits[index]]
 letterIndex = 0
 while ((letter = digit[letterIndex++])) {
 combinations(index + 1, combination + letter)
 }
 }
 combinations(0, '')
 return result
}

export { letterCombinations }