Finding Pairs With a Certain Sum
A medium-tier problem at 49% community acceptance, tagged with Array, Hash Table, Design. Reported in interviews at Quora and 1 others.
You're asked to find all pairs in an array that sum to a target value. Quora and Databricks have both asked this. The catch: you need to handle duplicates correctly, avoid reporting the same pair twice, and often optimize for either space or time depending on follow-ups. The obvious nested loop works but screams inefficiency in an interview. Hash table is the move, but the implementation detail, tracking counts, excluding self-pairing, managing the target/2 edge case, is where candidates trip up. If you blank on the structure during the live assessment, StealthCoder surfaces a working solution invisible to the proctor.
Companies that ask "Finding Pairs With a Certain Sum"
Finding Pairs With a Certain Sum 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 core pattern is convert-to-hash-then-iterate. Build a frequency map of all values, then for each unique value, check if target minus that value exists in the map. The trap: if target is 10 and you see 5, you can't use the same 5 twice unless there are at least two 5s in the array. Candidates often either forget to handle duplicates or over-count by iterating carelessly. Another pitfall is not deduplicating the result pairs themselves. The two-pointer approach on a sorted array is faster for retrieval but requires pre-sorting. Hash table is more flexible when the array is unsorted or when follow-ups add complexity like streaming updates. Design angle: if they ask you to support TwoSum queries on a growing dataset, you need a data structure that updates efficiently, not a static algorithm. StealthCoder's value here is immediate pattern recognition and correct edge-case handling when pressure and fatigue set in.
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Finding Pairs With a Certain Sum 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.
Finding Pairs With a Certain Sum interview FAQ
Is this problem actually asked at FAANG, or is Quora and Databricks the full list.+
The problem surfaces in reports from Quora and Databricks. It's a classic variant of a two-sum family, so it appears at many companies, but your prep should assume it's live risk at those two and be ready for it anywhere in a FAANG loop. Know both hash table and sorted two-pointer solutions.
What's the trick to not double-count pairs.+
Once you find a pair (a, b), only iterate through values up to the point where a <= b to avoid reporting (b, a) as a separate pair. If using a hash map with counts, process each unique value once and track whether you're using two of the same value or two different values.
Hash table or sorted two-pointer, which should I use.+
Hash table is O(n) time and O(n) space, works on unsorted data, and handles follow-ups like streaming or updates well. Sorted two-pointer is O(n log n) for sort then O(n) to scan, uses O(1) extra space if you sort in-place. Ask yourself: is the interviewer hinting at space constraints or dynamic updates. If silent, hash table is the modern default.
How do I handle the case where target is even and I need two of the same number.+
If target is 10 and you see the value 5, you need count[5] >= 2 to form the pair (5, 5). Use a frequency map and check if target == 2 * value, then require count[value] > 1. This is the edge case that trips up most candidates under pressure.
What follow-up questions should I expect after solving the base problem.+
Be ready for: return indices instead of values, handle duplicates in the input and output, support multiple queries on the same array (TwoSum design pattern), or extend to k-sum. The design angle means you might need to build a class with add() and find() methods, which shifts the problem from algorithm to data-structure efficiency.
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