https://leetcode.com/problems/letter-combinations-of-a-phone-number/

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/**
 * @param {string} digits
 * @return {string[]}
 */
const map = {
    2: 'abc',
    3: 'def',
    4: 'ghi',
    5: 'jkl',
    6: 'mno',
    7: 'pqrs',
    8: 'tuv',
    9: 'wxyz'
}
let ret = []

const letterCombinations = (digits) => {
    ret = []
    if (digits.length !== 0) {
        backtrack('', digits)
    }
    return ret
}

const backtrack = (comb, next) => {
    if (next.length === 0) {
        ret.push(comb)
    }
    else {
        const digit = next[0]
        const letters = map[digit]
        for (let i = 0; i < letters.length; i++) {
            const letter = letters[i]
            backtrack(comb + letter, next.substring(1))
        }
    }
}