Alternating Digit Sum
A easy-tier problem at 68% community acceptance, tagged with Math. Reported in interviews at eBay and 0 others.
Alternating Digit Sum is an easy problem that shows up in eBay assessments. With a 68% acceptance rate, it's the kind of problem that looks trivial until you miss the alternation pattern and spiral on the implementation. The trick is simpler than you think, but the devil is in the order of operations. If you blank on how to alternate signs during the live OA, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Alternating Digit Sum"
Alternating Digit 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 a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe problem asks you to compute a sum of digits where each digit's sign alternates (positive, negative, positive, etc.). Most candidates overthink it: they build the digit array, reverse it, then apply signs. Wrong. You either iterate forward and manage the sign yourself, or you iterate backward from the string's natural order. The pitfall is off-by-one errors in sign flipping. The obvious approach (extract digits naively, then alternate) works fine, but candidates often reverse when they shouldn't or apply the sign to the wrong position. This is where StealthCoder is the hedge: if you panic on sign logic during the assessment, you get a clean, tested implementation instantly.
Pattern tags
You know the problem.
Make sure you actually pass it.
Alternating Digit Sum 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 a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Alternating Digit Sum interview FAQ
Is Alternating Digit Sum really asked at eBay?+
Yes, eBay reports this problem in their assessment pool. It's an easy problem, so expect it early in their technical screen or as a warm-up. The 68% pass rate confirms it's not a trick question, but execution matters.
What's the actual trick to this problem?+
There is no trick. It's pure implementation discipline. Convert the number to a string or extract digits, iterate through them, and flip the sign at each step. The trap is sign management: starting with positive, alternating correctly, and not reversing the digit order unless the problem explicitly requires it.
Should I reverse the digits or iterate left to right?+
Read the problem statement carefully. Most versions ask you to alternate starting from the rightmost digit (units place). You can iterate the string backward or extract digits in reverse order, then apply signs. Either works. Consistency is what matters.
How does Alternating Digit Sum relate to the Math topic?+
It's pure arithmetic. No advanced number theory, no modulo tricks, no primes. You're just cycling through digit values and managing a running sum with alternating signs. Solid warm-up for mental math and loop discipline.
Will I see this problem at other companies?+
eBay is the only company currently reporting this specific problem in the dataset. It's not a common FAANG question, so it's lower-profile. That means less community discussion and fewer YouTube walkthroughs. A real edge case for live prep.
Want the actual problem statement? View "Alternating Digit Sum" on LeetCode →