Two Sum III - Data structure design
A easy-tier problem at 39% community acceptance, tagged with Array, Hash Table, Two Pointers. Reported in interviews at LinkedIn and 0 others.
Two Sum III hits different because it's not a one-pass algorithm problem, it's a design problem hiding inside a two-sum shape. You're building a data structure that has to handle a stream of additions and then answer repeated sum queries. LinkedIn asks this one, and the acceptance rate is under 39%, which tells you most candidates either over-engineer the storage or tank on query latency. The trick isn't the two-sum logic you've memorized, it's deciding what to optimize for: fast adds and slow queries, or slow adds and fast queries. If this lands in your OA and you freeze on the tradeoff, StealthCoder surfaces a working design in seconds, invisible to the proctor.
Companies that ask "Two Sum III - Data structure design"
Two Sum III - Data structure design 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 pattern here is that you're not solving two-sum once, you're solving it many times on a growing dataset. A hash table stores every number you've added, and when sum queries arrive, you iterate through stored values and check if the complement exists. This is O(n) per query but O(1) adds. The alternative is sorting on each query, which is O(n log n) but lets you use two pointers for elegance. Most candidates miss that the problem is really asking you to articulate the space-time tradeoff and justify your choice based on expected call patterns. The common miss is treating it like the classic two-sum problem and panicking when the interviewer pushes back on why you didn't just solve it that way. Arrays, hash tables, and design patterns all converge here. StealthCoder handles both approaches and shows you the cleaner implementation for whichever path you commit to.
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Two Sum III - Data structure design recycles across companies for a reason. It's easy-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.
Two Sum III - Data structure design interview FAQ
Why is the acceptance rate so low on this one?+
It's a design problem, not an algorithm problem. Most candidates know the two-sum trick but don't know how to architect a data structure that handles streaming input and repeated queries. The tradeoff between storage layout and query speed catches people off guard.
Do I need to handle duplicate numbers?+
The problem statement usually allows duplicates in the input and the sum query. That's why a frequency map (hash table with counts) often outperforms a simple set. One clean entry point for this question is to ask the interviewer upfront.
Hash table or sorted array, which is the 'right' answer?+
Neither. It depends on call patterns. If adds are rare and sum queries are frequent, sort on each query. If adds flood in and queries are sparse, use a hash table. A good answer explains the tradeoff, not a memorized solution.
How does this relate to the classic LeetCode two-sum problem?+
Classic two-sum solves once on a static array. This version solves the same question repeatedly on a growing stream. The shift from 'preprocess once' to 'handle live updates' changes which data structure wins.
Will LinkedIn really ask this at scale, or is it a warm-up?+
LinkedIn asks design problems to see how you think about tradeoffs under load. This one is easier than distributed system design but harder than raw algorithm problems. Expect follow-ups on memory and latency constraints once you've nailed the core structure.
Want the actual problem statement? View "Two Sum III - Data structure design" on LeetCode →