Missing Number In Arithmetic Progression
A easy-tier problem at 52% community acceptance, tagged with Array, Math. Reported in interviews at Audible and 0 others.
Missing Number in Arithmetic Progression is a deceptively simple problem that catches candidates off guard because the obvious brute-force approach works fine, but interviewers are fishing for the mathematical insight. You're given an array where all numbers follow an arithmetic sequence except one is missing, and you need to find it. Audible has asked this. With a 52% acceptance rate, half the people who attempt it don't nail it on first try. The trap: spending cycles sorting or iterating when the answer lives in basic algebra.
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Missing Number In Arithmetic Progression 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 trick is recognizing that in a perfect arithmetic progression, the difference between consecutive elements is constant. Instead of scanning for gaps, calculate what that difference should be by comparing the first and last element divided by the array length. Then iterate once to find which position breaks the pattern. The gotcha is off-by-one errors when the missing number is at the start or end, and floating-point division when the step size isn't clean. Most candidates default to sorting and linear search, which works but signals you didn't see the pattern. If you freeze on this during the live assessment, StealthCoder surfaces the mathematical approach instantly, invisible to the proctor.
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Missing Number In Arithmetic Progression 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 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.
Missing Number In Arithmetic Progression interview FAQ
Is this problem actually easy, or is 52% acceptance misleading?+
It's easy in structure, hard in execution. The concept is simple arithmetic, but candidates fumble the index math or forget to handle boundary cases. The 52% rate reflects careless implementation, not algorithmic complexity.
Will I see this at other companies, or is it Audible-specific?+
This is a classic array and math problem, so it circulates. Audible is reported, but expect similar missing-element variants across tech hiring. The pattern matters more than the specific problem.
Do I need to sort the array?+
No. Sorting wastes O(n log n) time. The array is already in sequence; use that fact. Calculate the step from endpoints and verify it in a single pass.
What's the edge case most people miss?+
When the missing number is the first or last element, your index logic flips. Test both. Also test arrays where the step size is negative, or where all remaining numbers are equal distance apart.
How does this relate to the array and math topics listed?+
Array because you're iterating and indexing. Math because the solution is arithmetic progression theory, not clever data structures. Knowing the formula beats knowing a trick.
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