Largest Palindromic Number
A medium-tier problem at 36% community acceptance, tagged with Hash Table, String, Greedy. Reported in interviews at Bentley Systems and 2 others.
Largest Palindromic Number is a medium-difficulty string problem that hits roughly one-third of candidates. It's been asked at Bentley Systems, Geico, and SmartNews. The problem sounds simple: build the largest palindrome from digits in a given string. But the greedy intuition most people reach for will fail. You need to think about digit pairing and center placement strategically. If this problem appears in your live OA and the greedy approach crumbles, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Largest Palindromic Number"
Largest Palindromic 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 by a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe trap is treating this as a pure greedy problem. You can't just stack the largest digits on the outside and call it done. The real pattern involves counting digit frequencies, pairing them symmetrically, and understanding that a palindrome reads the same forwards and backwards. This means you build half the number, mirror it, and handle the middle digit carefully if one remains unpaired. Hash Table lets you count occurrences efficiently. Greedy determines digit order. The trick is recognizing that leading zeros break the answer, and edge cases like all zeros or single digits need special handling. Most failures come from mishandling the center element or forgetting the zero-padding rule. StealthCoder is your hedge if the counting-plus-pairing pattern doesn't click during the timed assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Largest Palindromic 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 by a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Largest Palindromic Number interview FAQ
Why doesn't a pure greedy approach work here?+
Greedy alone doesn't account for palindrome symmetry. You can't just place digits left-to-right without ensuring the structure mirrors. You must count digit pairs, place them symmetrically, then decide which unpaired digit goes in the middle. The constraint is structural, not just ordering.
Is this really asked at major tech companies?+
Yes. Bentley Systems, Geico, and SmartNews have all asked it. It's not as common as array or tree problems, but it pops up often enough that ignoring it in prep is risky. The 36% acceptance rate shows it punishes candidates who haven't seen the pattern.
What's the core algorithmic insight?+
Count digit frequencies using a Hash Table, pair them up symmetrically, pick the largest unpaired digit for the center, then assemble the left half, center, and mirrored right half. Leading zeros must be handled to avoid invalid results like '000'.
How does this relate to the topics listed?+
Hash Table counts digits. String builds the result. Counting is the explicit step. Greedy selects which digits to place first. All four topics work together. You can't skip any part without the solution breaking.
What are the most common failure modes?+
Forgetting to handle leading zeros, not pairing digits correctly, placing the center element wrong, or treating it as a pure ordering problem. Most people also miss that you build only half the string and mirror it, then optimize center selection.
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