Number of Divisible Substrings
A medium-tier problem at 73% community acceptance, tagged with Hash Table, String, Counting. Reported in interviews at Amdocs and 3 others.
Number of Divisible Substrings hits harder than it looks. You'll see substring counting problems across companies, and this one appears in OAs at Amdocs, Wayfair, Paytm, and IBM. The 73% acceptance rate is misleading because most candidates either brute-force into a timeout or miss the mathematical pattern entirely. If you blank on the trick during your live assessment, StealthCoder solves it in seconds invisible to the proctor. This problem is a teach-yourself moment for prefix sums and hash tables working together.
Companies that ask "Number of Divisible Substrings"
Number of Divisible Substrings 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. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe naive approach counts substrings by checking divisibility of each one. That works until the input scales, then it times out. The real solution uses a prefix sum to track remainders, then counts matching remainders in a hash table. You're not actually computing substring values, you're tracking the remainder pattern and solving a "how many pairs have the same remainder" problem. Hash Table and Prefix Sum aren't separate topics here, they're a team. Most candidates see "divisible" and try arithmetic first. The trick is recognizing this as a remainder problem. StealthCoder is your insurance if the pattern doesn't click under pressure.
Pattern tags
You know the problem.
Make sure you actually pass it.
Number of Divisible Substrings 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. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Number of Divisible Substrings interview FAQ
Is this actually a medium problem?+
The 73% acceptance rate says yes, but that includes people who've solved remainder-tracking problems before. If prefix sum and modular arithmetic aren't instinctive to you yet, expect it to feel harder on first pass. Companies ask it because it teaches a real pattern.
Why do hash tables matter for divisibility?+
You're not storing divisibility checks. You're storing remainder frequencies. Two substrings are divisible by k if their prefix sums have the same remainder modulo k. Hash table lets you count matching remainders in O(1) per character instead of checking every substring pair.
Is this still asked at these companies?+
Amdocs, Wayfair, Paytm, and IBM all report it. That's a narrow but real list. If you're interviewing at any of them, expect substring and counting problems in that difficulty band.
What's the biggest trap?+
Trying to build and check substrings one by one. You'll TLE on large inputs. The shift to prefix remainder tracking is non-obvious if you haven't seen it. Spend time understanding why two indices with the same remainder mod k guarantees a divisible substring.
Does this relate to other counting problems?+
Yes. It's a sibling to Subarray Sum Equals K and similar prefix-hash problems. If you know that pattern, this clicks faster. If not, it's a good template to learn once and reuse across interviews.
Want the actual problem statement? View "Number of Divisible Substrings" on LeetCode →