161. Letter Combinations of a Phone Number

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Input: digits = "23"

Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Explanation: The digit '2' maps to 'abc' and '3' maps to 'def'. The output contains all possible combinations.

Input: digits = "2"

Output: ["a","b","c"]

Explanation: The digit '2' maps to 'abc', so the output contains all those individual letters.

Time Complexity

O(4^N × N)

Space Complexity

O(N)