T9 Keypad Mapping | Dataford Interview Questions

How would you convert a string of digits into the corresponding T9 text message sequences?

Implement letter_combinations(digits) using the standard phone keypad mapping from digits 2 through 9 to letters. Return all possible letter strings in depth-first generation order. For an empty input, return an empty list.

Examples: "23" returns ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]; "7" returns ["p", "q", "r", "s"].

Constraints: 0 <= len(digits) <= 8; every character is a digit from 2 through 9.

Constraints

0 <= len(digits) <= 8
Every character in digits is between '2' and '9'
Return sequences in depth-first keypad order
The output contains the Cartesian product of the mapped letters

Function Signature

def letter_combinations(digits):

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