17. Letter Combinations of a Phone Number
MediumBacktracking
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order. A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters. 2: abc, 3: def, 4: ghi, 5: jkl, 6: mno, 7: pqrs, 8: tuv, 9: wxyz.
Examples
Input: 23
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]
Explanation: Digit 2 -> a/b/c, digit 3 -> d/e/f; all 9 pairings.
Constraints
- 0 <= digits.length <= 4
- digits[i] is a digit in the range ['2', '9'].
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