Longest Palindromic Substring
A medium-tier problem at 36% community acceptance, tagged with Two Pointers, String, Dynamic Programming. Reported in interviews at CEDCOSS and 73 others.
Longest Palindromic Substring is a medium-difficulty string problem that appears across 74 companies, including Cisco, Zoho, Dell, and Wix. You need to find the longest contiguous substring that reads the same forwards and backwards. The 35% acceptance rate signals that most candidates either brute-force it (too slow) or freeze on the DP formulation mid-interview. If this problem hits your live assessment and you blank on the expansion or DP states, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Longest Palindromic Substring"
Longest Palindromic Substring 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 an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe trap is overthinking the approach. Brute force checks every substring (O(n^3)), which fails on large inputs. Two-pointer expansion from each center is cleaner and runs O(n^2). Dynamic programming also works (O(n^2) time, O(n^2) space) but requires careful state definition: dp[i][j] = true if substring from index i to j is a palindrome. The DP transition hinges on checking if s[i] == s[j] and whether the inner substring is palindromic. Most failures come from off-by-one errors or not handling odd/even-length palindromes correctly in the expansion approach. StealthCoder is your safety net if the expansion logic trips you up under pressure.
Pattern tags
You know the problem.
Make sure you actually pass it.
Longest Palindromic Substring 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 an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Longest Palindromic Substring interview FAQ
Is this actually asked at big companies or just small ones?+
It's in the rotation at scale. Dell, Cisco, Wix, and RBC all report it. Not a gimme, but it's a known pattern. If you see it, the company knows you either studied or got lucky. Prepare for follow-ups on space optimization.
What's the trick that separates the 35% who solve it?+
Center expansion, not DP. Candidates who jump straight to DP often botch the state transitions or array indexing. Expanding from each center (treating odd and even lengths separately) is faster to code and debug. It's the pattern that clicks in real time.
Should I use Two Pointers or Dynamic Programming?+
Two Pointers is faster to implement and equally efficient at O(n^2). DP is more generalizable to related problems but adds cognitive load in the live assessment. Unless the interviewer explicitly asks for DP, expand from centers. Both topics are tagged here, so interviewers know both work.
What happens if the string has no palindrome longer than one character?+
You still return a single character. Any string of length 1 is a palindrome. The algorithm defaults to the first character if no longer substring is found. Not a gotcha, but confirm your loop handles empty strings or length-1 inputs gracefully.
How much time do I actually have to solve this in an OA?+
Most assessments give 30-45 minutes for a medium. Write the center-expansion solution in 15-20 minutes, test on 2-3 cases (including edge cases like single char and all same char), then move on. If you hit a wall, StealthCoder delivers a working solution so you don't tank the rest of the OA.
Want the actual problem statement? View "Longest Palindromic Substring" on LeetCode →