4Sum
A medium-tier problem at 38% community acceptance, tagged with Array, Two Pointers, Sorting. Reported in interviews at oyo and 20 others.
4Sum shows up in assessments at Amazon, Google, Microsoft, Apple, and Bloomberg. It's a medium-difficulty problem with a low acceptance rate around 38 percent, which means most candidates either time out, miss edge cases, or don't see the pattern. The obvious brute force (four nested loops) times out instantly. The trick is sorting plus two pointers, but the real catch is handling duplicates without returning the same quadruplet twice. If this problem hits your live OA and you blank on the approach, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "4Sum"
4Sum 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 StealthCoder4Sum is a constraint-satisfaction problem disguised as a search problem. Sort the array first, then use two nested loops to pick the first two elements, and two pointers to close the gap on the remaining window. The pattern mirrors 3Sum (which is itself Two Sum with a loop wrapper), so the reduction feels clean once you see it. The trap: candidates often hardcode the first loop incorrectly or fail to skip duplicates at the right step, breaking the output. Edge cases like negative numbers, zeros, and multiple identical quadruplets break half the submissions. The two-pointer technique keeps complexity at O(n cubed), which passes. StealthCoder is the hedge if you drill the pattern once but forget the duplicate-skipping logic under pressure.
Pattern tags
You know the problem.
Make sure you actually pass it.
4Sum 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.
4Sum interview FAQ
Is 4Sum really asked at FAANG companies?+
Yes. It appears in assessments at Amazon, Google, Microsoft, Apple, and Bloomberg. The 38 percent acceptance rate reflects the duplicate-handling complexity and the need to avoid the brute-force O(n^4) trap. Companies use it to filter for candidates who understand sorting plus two-pointer optimization.
What's the main trick I'm missing?+
Sort first, then fix two elements with nested loops and close with two pointers on the remainder. The real trick is skipping duplicates without breaking the logic. Most candidates either over-skip or under-skip, returning dupes or missing valid quadruplets. Practice the skip-after-a-match pattern.
How does 4Sum relate to 3Sum and Two Sum?+
Two Sum uses two pointers on a sorted array. 3Sum wraps a loop around Two Sum. 4Sum wraps another loop around 3Sum. Once you own the two-pointer base, the pattern stacks. The difficulty isn't the algorithm; it's nesting and duplicate handling without losing your place.
Why is acceptance rate so low?+
Edge cases: zeros, negatives, repeated elements, and the need to return unique quadruplets. Candidates either return duplicates or time out on brute force. The two-pointer optimization isn't obvious to everyone, and duplicate-skipping requires careful index management. Missing one step breaks the solution.
What's the time complexity I need to hit?+
O(n cubed) is the target. Sort is O(n log n), then two nested loops with a two-pointer scan inside. O(n^4) brute force will time out. If you're hitting that wall, you're missing the two-pointer reduction. Space is O(1) if you ignore sort space, or O(n) if you count it.
Want the actual problem statement? View "4Sum" on LeetCode →