Length of Longest Fibonacci Subsequence
A medium-tier problem at 58% community acceptance, tagged with Array, Hash Table, Dynamic Programming. Reported in interviews at Baidu and 1 others.
You're asked to find the longest subsequence of an array that forms a valid Fibonacci sequence. This problem appears in assessments at Baidu and Goldman Sachs, which means it's testing whether you can spot a hidden numerical pattern in an unsorted collection. The trick isn't sorting or greedy selection. It's recognizing that any two elements can seed a Fibonacci chain, and you need to track those chains as you iterate. With a 58% acceptance rate, most candidates either brute-force incorrectly or miss the dynamic programming insight entirely. If you hit this live and blank on the pattern, StealthCoder runs invisibly during your assessment and surfaces the working solution.
Companies that ask "Length of Longest Fibonacci Subsequence"
Length of Longest Fibonacci Subsequence 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. Made by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too.
Get StealthCoderThe naive instinct is to generate Fibonacci sequences and check membership, but that explodes in complexity. The real approach: for each pair of elements (a, b), treat them as the first two terms of a potential Fibonacci sequence, then check if subsequent Fibonacci numbers exist in the array. You'll use a hash table to store elements for O(1) lookup and dynamic programming to track the length of each sequence seeded by that pair. The gotcha is handling the DP state correctly, you need to map (previous_two_elements) to (sequence_length) so you can extend chains as you scan forward. Many candidates fail because they iterate naively or miscalculate the next Fibonacci value. Hash Table and Dynamic Programming are both core here. StealthCoder is the hedge if the nested DP logic tangles you mid-assessment.
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Length of Longest Fibonacci Subsequence 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. Made by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Length of Longest Fibonacci Subsequence interview FAQ
Is this problem really asked at top companies?+
Yes. Baidu and Goldman Sachs both report it. It's not a household name like Two Sum, but it signals whether you can handle pattern recognition under DP constraints. The 58% pass rate suggests it filters well.
What's the actual trick I'm missing?+
Any two distinct array elements can start a Fibonacci sequence. You iterate through the array and for each element, check all prior pairs to see if this element extends their chain. Use a hash table for O(1) lookups and a DP map keyed by (second_last, last) to track sequence lengths.
Why doesn't brute-force work?+
Generating all Fibonacci numbers up to max(array) and checking membership is slow if max(array) is large. You'd generate exponentially many sequences. The DP+hash approach is O(n^2) because you check each of the O(n^2) pairs exactly once.
How do Array, Hash Table, and DP fit together?+
Array is the input. Hash Table gives you O(1) membership checks for candidate next values. DP memoizes the longest chain ending at each pair of consecutive Fibonacci terms, so you don't recompute.
What's a common mistake in the DP state?+
Keying the DP map by a single element instead of a pair. A Fibonacci sequence is defined by its last two terms, not one. If you lose that context, you'll overwrite lengths or miss chains.
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