Letter Combinations of a Phone Number
Given a string containing digits 2-9, return all possible letter combinations Mapping: 2->abc, 3->def, 4->ghi, 5->jkl, 6->mno, 7->pqrs, 8->tuv, 9->wxyz Example: digits = "23" => ["ad","ae","af","bd","be","bf","cd","ce","cf"]
This one is about exploring choices without getting lost—choose, recurse, undo, repeat. Interviewers love it because it reveals your structured exploration and your edge-case discipline. If you can explain your pruning, this stops feeling like brute force.
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