Next Palindromic Time
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
The mistake that sinks a first attempt on this Amazon question, reported in September 2026, is returning the input time when it's already a palindrome. The prompt says strictly later, and that one word fails the edge case. The task is simple: given HH:MM, find the next time whose four digits read the same both ways, wrapping past 23:59. It's a simulation problem with a tiny search space. You can brute force it in under 1440 steps. If you blank during the live OA, StealthCoder runs invisibly on your desktop as a safety net, so one missed detail doesn't cost you the round.
The problem
Given a valid 24-hour time time in HH:MM format, return the first strictly later time whose four digits form a palindrome. Advance in one-minute steps and wrap from 23:59 to 00:00. The input itself is never returned unless a full day has elapsed. Function nextPalindromicTime(time: String) → String Examples Example 1 time = "12:21" return = "13:31" 13:31 is the first later valid time whose digits read the same in both directions. Example 2 time = "23:32" return = "00:00" No later palindromic time remains that day, so the search wraps to midnight. Example 3 time = "05:50" return = "10:01" The next palindromic four-digit clock value after 0550 is 1001. Constraints time.length == 5. time[2] == ':'. time is a valid zero-padded 24-hour time from 00:00 through 23:59.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is that you don't need cleverness. Convert the time to total minutes, then loop from 1 to 1440. At each step compute (start + i) % 1440, turn it back into hours and minutes, and check whether the digits h1 h2 m1 m2 satisfy h1 == m2 and h2 == m1. Return the first hit. Starting the loop at 1 handles the strictly later rule. Running to 1440 covers the case where the input is the only palindrome you'd ever see, which comes back after a full day. That case can't really occur here, since many palindromic times exist, but the loop bound is still correct. Pitfalls: forgetting zero-padding on output, so 0:00 prints wrong, and checking only the hour string against its reverse. Also don't forget the wrap like 23:32 to 00:00. If the live assessment rattles you, StealthCoder can supply this loop in real time as a hedge.
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Next Palindromic Time FAQ
How hard is Next Palindromic Time really?+
Easy. The search space is at most 1440 minutes, so brute force passes. The difficulty is in the details: strictly later, midnight wrap, and zero-padded output. Candidates fail on those, not on the algorithm.
What's the trick to solve it fast?+
Convert to minutes since midnight, loop i from 1 to 1440, take (start + i) % 1440, format as HH:MM, and check that the four digits form a palindrome. Return on the first match. No math tricks are needed.
Should I return the input if it's already a palindrome?+
No. The problem says strictly later. For 12:21 the answer is 13:31, not 12:21. Start your loop at one minute ahead so the input never matches itself, unless a full day passes.
How do I handle the wrap from 23:59 to 00:00?+
Use modulo 1440 on the total minutes. Then hours are minutes divided by 60 and minutes are the remainder. Example 2 shows 23:32 giving 00:00, which comes out naturally from the modulo with no special case.
How do I prepare for this in 48 hours?+
Write the loop once from scratch and test the three given examples. Then add edge cases like 23:59, 00:00, and 09:59. Practice zero-padded formatting in your language, since that's where silent bugs show up.