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):

• 2 → abc
• 3 → def
• 4 → ghi
• 5 → jkl
• 6 → mno
• 7 → pqrs
• 8 → tuv
• 9 → wxyz

Note that 1 does not map to any letters.

Examples

Example 1

Input: digits = "23"

Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Explanation: Digit 2 maps to "abc" and digit 3 maps to "def", giving 9 combinations.

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']