Double Modular Exponentiation
A medium-tier problem at 47% community acceptance, tagged with Array, Math, Simulation. Reported in interviews at Barclays and 0 others.
Double Modular Exponentiation is a medium-difficulty problem that combines modular arithmetic with iteration over an array. With a 47% acceptance rate, it trips up candidates who either misunderstand the modular properties or implement exponentiation naively. Barclays has asked this, which signals it's screening for candidates who can handle both the math and the implementation cleanly. The problem feels straightforward until you hit the constraint: brute force modular exponentiation will timeout. If this lands in your live assessment and you blank on the optimization, StealthCoder runs invisible behind your screen and surfaces a working solution in seconds.
Companies that ask "Double Modular Exponentiation"
Double Modular Exponentiation 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 who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe core trick is recognizing that you can't just compute large powers and then mod them. You must apply the modulus at each step using fast exponentiation (or repeated squaring). Many candidates write a naive loop that multiplies and mods each time, which works for small inputs but fails on large exponents. The pattern: break the problem into array iteration plus modular exponentiation. Common pitfall is forgetting that (a * b) mod m = ((a mod m) * (b mod m)) mod m, which lets you keep intermediate results small. The simulation aspect means you're applying this logic repeatedly across array elements. StealthCoder hedges the case where you recognize the array loop but freeze on the exponentiation detail during the real assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Double Modular Exponentiation 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 who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Double Modular Exponentiation interview FAQ
How hard is this really compared to standard LeetCode medium?+
The 47% acceptance rate suggests it's genuinely tricky. Most medium problems land 50-60%. This one punishes brute-force exponentiation and requires you to know fast exponentiation cold. It's a real medium, not an easy dressed up.
Is Barclays still asking this in their OAs?+
We have one report of Barclays asking it. Finance firms do ask modular arithmetic problems because they care about both math rigor and clean coding. Assume it's in their rotation.
What's the trick I'm missing if I timeout?+
You're doing full exponentiation before modding, or using Python's ** operator without built-in modular exponentiation. Use fast exponentiation (exponentiation by squaring) or pow(base, exp, mod) in Python, which is O(log exp) not O(exp).
Does this relate to other array + math problems I should drill?+
Yes. It combines array iteration with modular arithmetic. If you've solved problems on prefix sums or array simulation, the loop structure is familiar. The new thing is the modular exponentiation itself, so that's what to isolate and drill.
Will I see this at other companies, or just Barclays?+
Modular exponentiation is a classic interview topic, especially at firms doing crypto, finance, or backend systems work. One company in our data doesn't mean it's exclusive. Treat this as a core pattern worth knowing cold for any technical OA.
Want the actual problem statement? View "Double Modular Exponentiation" on LeetCode →