Letter Combinations of a Phone Number

Problem Statement
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. A mapping of digits to letters (just like on the telephone buttons) is given below. Return the answer in any order.[1][9]

Digit Mapping

• 2: abc
• 3: def
• 4: ghi
• 5: jkl
• 6: mno
• 7: pqrs
• 8: tuv
• 9: wxyz[2][1]

Input/Output Examples

• Input: digits = "23"
  Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"][1][2]
• Input: digits = ""
  Output: [][2][1]
• Input: digits = "2"
  Output: ["a","b","c"][2]

Constraints

• 0 <= digits.length <= 4[1]
• digits[i] is a digit in the range ['2', '9'].[1]