Letter Combinations of a Phone Number
A medium-tier problem at 64% community acceptance, tagged with Hash Table, String, Backtracking. Reported in interviews at Epic Systems and 36 others.
You get a string of digits 2-9 and need to return all possible letter combinations, like a phone keypad. This problem shows up across 37 companies including Amazon, Tesla, and Epic Systems. It's a classic backtracking warm-up that feels easy until you're live and blank on the keypad mapping or how to build the combinations without nesting loops forever. The 64% acceptance rate masks a lot of candidates who code it the hard way or forget edge cases. If you haven't drilled the pattern and it lands in your assessment, StealthCoder solves it in seconds invisible to the proctor, so you can move on.
Companies that ask "Letter Combinations of a Phone Number"
Letter Combinations of a Phone Number is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe trick is mapping each digit to its letters, then using backtracking to build every valid combination. Most candidates either hardcode the mapping poorly or try nested loops and get tangled. The real pattern: start with an empty combination, iterate through the input digits, and for each digit append every letter from its keypad group to every existing combination. You end up with a list that grows exponentially but stays clean. Hash Table holds the digit-to-letters mapping, String is what you're combining, Backtracking is the control flow. The gotcha is handling empty input and single digits without crashing. If you freeze on the recursion shape during a live OA, StealthCoder is the safety net that surfaces a working solution while you stay calm.
Pattern tags
You know the problem.
Make sure you actually pass it.
Letter Combinations of a Phone Number recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Letter Combinations of a Phone Number interview FAQ
Is this problem still asked at big companies?+
Yes. It appears in 37 company reports, including Amazon, Tesla, and Epic Systems. It's a screening favorite because it's medium difficulty, quick to code if you know the pattern, and exposes messy thinking fast. Expect it as a warm-up or second pass.
What's the trick to not overcomplicating it?+
Precompute the keypad mapping (digit to letters) in a hash table, then iteratively or recursively build combinations by appending each letter from the current digit's group to all prior combinations. Avoid nested loops. One pass through the digits, one pass per combination.
How does backtracking apply here?+
You explore every path (digit-to-letter choice) and prune or unwind when you've consumed all input digits. The recursion depth equals input length, making it clean and efficient. It's a canonical backtracking drill that also works iteratively if recursion isn't your style.
What edge cases sink candidates?+
Empty input (return empty list, not list with empty string), single digit, and remembering the digit 0 and 1 have no letters on a phone keypad. Test those three before submitting. Most rejects happen on empty input handling.
How hard is it really, given 64% acceptance?+
The algorithm is straightforward once you visualize it, but in live conditions, 36% of people falter on the mapping, recursion shape, or edge cases. It's a precision problem, not a hard problem. Clean implementation matters more than clever logic.
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