LeetCode-in-All

17. Letter Combinations of a Phone Number

Medium

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'].

Solution

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function (digits) {
    if (digits.length === 0) {
        return []
    }

    const letters = ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz']
    const result = []
    const current = []

    function findCombinations(index) {
        if (current.length === digits.length) {
            result.push(current.join(''))
            return
        }

        for (let i = index; i < digits.length; i++) {
            const digit = parseInt(digits[i])
            const letterGroup = letters[digit]

            for (const char of letterGroup) {
                current.push(char)
                findCombinations(i + 1)
                current.pop()
            }
        }
    }

    findCombinations(0)
    return result
}

export { letterCombinations }

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