Minimum Absolute Pair Difference
Reported by candidates from Upstart's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Upstart reported this one in July 2026, and it's less scary than the title sounds. Strip the wording and it's one move: sort the array, then compare neighbors. The minimum absolute difference between any two elements always sits between two adjacent values in sorted order. With 200000 elements, the brute-force pair check is the trap, and you know it. If you blank on the sort idea during the OA, StealthCoder is the safety net running invisibly on your screen. But you probably won't need it.
The problem
Given an unordered integer array values, choose two elements at different indices and return the minimum possible absolute difference between their values. Equal values at different indices are a valid pair and produce a minimum difference of 0. Function minimumDifference(values: int[]) → long Examples Example 1 values = [1,20,100,3] return = 2 The closest pair is 1 and 3, whose absolute difference is 2. Example 2 values = [8,-4,12,-5] return = 1 The pair -5 and -4 has difference 1. Example 3 values = [7,2,7] return = 0 The two occurrences of 7 are at different indices, so they form a valid pair with difference 0. Constraints 2 <= values.length <= 200000 -10^9 <= values[i] <= 10^9 The result fits in a signed 64-bit integer.
Reported by candidates. Source: FastPrep
Pattern and pitfall
The trick is that the closest pair is always adjacent after sorting. Sort a copy of values, loop from index 1 to the end, and track the smallest gap between values[i] and values[i-1]. That's O(n log n) time. Duplicates fall out for free, since equal neighbors give a gap of 0, which matches Example 3 with [7,2,7]. The common pitfall is the O(n^2) double loop, which dies at 200000 elements. The second pitfall is overflow. Values run from -10^9 to 10^9, so a difference can reach 2*10^9, which breaks a 32-bit int. Use a 64-bit type for the subtraction and the result. Initialize the answer to a huge value, not 0. If the sort idea slips away mid-OA, StealthCoder can hand you the clean version while you keep your head straight.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Minimum Absolute Pair Difference cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it.
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This OA pattern shows up on LeetCode as minimum absolute difference. If you have time before the OA, drill that.
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Upstart reuses patterns across OAs. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Minimum Absolute Pair Difference FAQ
What's the trick in Minimum Absolute Pair Difference?+
Sort the array first. After sorting, the closest two values must be next to each other, so you only compare adjacent pairs and keep the smallest gap. That turns an O(n^2) pair search into O(n log n), which is what 200000 elements needs.
How hard is this Upstart OA question really?+
Easy. It's a sort plus one linear pass. The difficulty is only in spotting that you don't need all pairs, and in remembering the overflow edge. If you've seen the closest-pair-in-an-array idea before, you can write it in a few minutes.
Do I need to worry about integer overflow?+
Yes. Values go from -10^9 to 10^9, so the gap between the smallest and largest can hit 2*10^9, which overflows a signed 32-bit int. The function returns long for that reason. Cast to a 64-bit type before subtracting, even though the minimum gap is usually small.
How do duplicates affect the answer?+
Duplicates at different indices count as a valid pair with difference 0. After sorting they sit side by side, so your adjacent comparison catches them automatically. You can also exit early the moment a gap of 0 shows up, since nothing beats it.
How should I prepare for this in 48 hours?+
Write the sorted-neighbors solution once from memory in your language of choice. Then test it on the three examples, including [7,2,7] and negatives like [8,-4,12,-5]. Check your answer variable uses a 64-bit type and starts at a large value. That's the whole prep.