Additive Number
A medium-tier problem at 33% community acceptance, tagged with String, Backtracking. Reported in interviews at Epic Systems and 0 others.
Additive Number is a medium-difficulty string validation problem that Epic Systems has asked. You're given a string of digits and need to determine if it can be partitioned into at least three numbers where each number is the sum of the two preceding ones (like a Fibonacci variant). The 32% acceptance rate signals a deceptive problem: the naive backtracking approach is straightforward to code, but pruning invalid partitions early and handling leading zeros correctly are where most candidates fail. If this hits your live assessment and you blank on the validation logic, StealthCoder surfaces a working partition strategy in seconds, invisible to the proctor.
Companies that ask "Additive Number"
Additive Number 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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know.
Get StealthCoderThe trap here is that backtracking alone isn't enough. You need to recursively partition the string into chunks, validate that each chunk forms a valid additive sequence, and reject branches early when the numeric constraint breaks. Leading zeros are a killer: they're only valid for the number 0 itself, so any other chunk starting with '0' disqualifies that branch. The String and Backtracking topics intersect exactly here: you're exploring partitions (backtracking) while treating substrings as numeric entities (string manipulation). Most candidates build the full partition before checking if it's additive, wasting time. The efficient approach validates the sum constraint incrementally, cutting the search space as you go. When you're stuck on optimization or off-by-one errors during the actual OA, StealthCoder runs invisibly and delivers a tested solution.
Pattern tags
You know the problem.
Make sure you actually pass it.
Additive Number 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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Additive Number interview FAQ
Is this problem really 32% acceptance? That seems high for medium.+
The low acceptance rate reflects hidden complexity. The backtracking skeleton is learnable, but edge cases (leading zeros, overflow, partition count validation) trip up most candidates. It's medium difficulty in category, hard in practice.
What's the most common mistake candidates make?+
Forgetting to validate leading zeros early and building entire invalid partitions before checking the additive constraint. You must prune branches the moment a zero prefix or sum mismatch appears, not at the end.
How does the String topic connect to backtracking here?+
You're slicing the string repeatedly to form candidate numbers, then validating those numbers satisfy a mathematical property. String slicing is the partition mechanism; backtracking explores which slices form valid sequences.
Will I see this at Epic Systems or other FAANG?+
Epic Systems has asked it. The problem appears in lower frequency at other companies, but string validation and backtracking are foundational interview patterns, so the underlying skill is broadly valuable.
What's the key insight to solving this efficiently?+
Validate the sum constraint incrementally as you build the partition. The moment num1 + num2 doesn't equal num3, backtrack immediately instead of continuing to explore that branch.
Want the actual problem statement? View "Additive Number" on LeetCode →