Interview question: Letter Combinations of a Phone Number

Problem Description

Given a string digits containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. The mapping of digits to letters is as follows:

{
"2": "abc",
"3": "def",
"4": "ghi",
"5": "jkl",
"6": "mno",
"7": "pqrs",
"8": "tuv",
"9": "wxyz"
}

Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit from '2' to '9'.