Example 1:
Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Example 2:
Input: digits = ""
Output: []
Example 3:
Input: digits = "7"
Output: ["p","q","r","s"]
Given a string digits containing digits from 2 to 9, return every possible letter combination the number could represent.
Use the classic phone keypad mapping where 2 maps to "abc", 3 maps to "def", 4 maps to "ghi", 5 maps to "jkl", 6 maps to "mno", 7 maps to "pqrs", 8 maps to "tuv", and 9 maps to "wxyz".
Return the combinations in any order. If digits is empty, return an empty array.
0 <= digits.length <= 4digits[i] is a digit in the range ["2", "9"].Reported as a standard backtracking question with follow-up discussion around recursion structure and iterative generation tradeoffs.