Practice/ServiceNow/Leetcode 17. Letter Combinations of a Phone Number

Leetcode 17. Letter Combinations of a Phone Number

CodingMust

Problem

On a classic telephone keypad, each digit from 2 through 9 is associated with a set of letters. Your task is to generate all possible letter combinations that can be formed by pressing a sequence of these digits.

Given a string containing digits from 2 to 9 inclusive, return all possible letter combinations that the number could represent in any order.

The digit-to-letter mapping follows the standard phone keypad layout:

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

Requirements

1. Return all possible letter combinations for the given digit string
2. The order of combinations in the output does not matter
3. If the input is an empty string, return an empty list
4. Each digit in the input will be between 2 and 9 inclusive

Constraints

• 0 ≤ digits.length ≤ 4
• Each character in digits is a digit in the range ['2', '9']
• The solution should handle all valid inputs efficiently

Examples

Example 1:

Input: digits = "23" Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"] Explanation: The combinations are formed by taking one letter from digit 2 (a, b, or c) and one letter from digit 3 (d, e, or f).

Example 2:

Input: digits = "" Output: [] Explanation: No digits means no combinations to generate.

Example 3:

Input: digits = "2" Output: ["a","b","c"] Explanation: Digit 2 corresponds to three letters on the keypad.

Example 4:

Input: digits = "79" Output: ["pw","px","py","pz","qw","qx","qy","qz","rw","rx","ry","rz","sw","sx","sy","sz"] Explanation: Digits 7 and 9 each map to 4 letters, creating 16 total combinations.