Count Integer Sequences
Reported by candidates from Xperi's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
You've got a Xperi OA coming up with a sequence counting problem, and the problem text isn't available yet. This is classic combinatorics wrapped in a tight time constraint. The pattern almost always boils down to either dynamic programming or a closed-form math solution. You're looking at building sequences under constraints, then tallying valid ones. StealthCoder becomes your safety net if the recursion tree gets tangled during the live assessment. Know the base cases cold and you're halfway there.
Pattern and pitfall
Sequence counting problems at Xperi typically ask you to enumerate valid integer sequences given constraints like length, sum, digit bounds, or ordering rules. The trap is brute force: generate every sequence and filter. The win is recognizing the recurrence relation. You'll usually define a state (current position, current sum, remaining length, last digit) and count paths to valid endpoints using memoization or bottom-up DP. The math shortcut is spotting when the answer is binomial coefficients or a closed formula. Common mistake: forgetting that sequences can start with zero, or missing negative integers if they're allowed. During the OA, if your DP recurrence feels bloated, step back and reframe the state space. StealthCoder helps you validate your recurrence against the problem constraints in real time.
Drill it cold or hedge it with StealthCoder. Either way, don't walk into the OA hoping you remember the trick.
You can drill Count Integer Sequences cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Made for the candidate who got the OA invite this morning and has 72 hours, not six months.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as combination sum iv. If you have time before the OA, drill that.
You've seen the question.
Make sure you actually pass Xperi's OA.
Xperi reuses patterns across OAs. Made for the candidate who got the OA invite this morning and has 72 hours, not six months. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Count Integer Sequences FAQ
What's the actual trick in sequence counting?+
Stop thinking 'generate all sequences.' Think 'how many ways can I reach a valid end state from each starting point.' Build a DP table where each cell represents a partial sequence state. Fill it bottom-up or use memoized recursion. The trick is picking the right state representation (position, sum, last value, etc.) so the recurrence is clean.
How do I know if it's a math formula versus DP?+
Small constraints (n less than 20) usually point to DP or even brute force. Large constraints (n over 100,000) almost always need a closed form or a derivable formula. If the problem hints at 'how many ways' without complex ordering rules, consider binomial coefficients or stars-and-bars. Test your DP on the examples first.
What's the most common edge case in Xperi OAs like this?+
Off-by-one errors in sequence length and forgetting the zero state. If sequences are 1-indexed, your DP should reflect that. Also watch whether the problem allows empty sequences or requires at least one element. Read the examples twice before coding.
Can I brute force this in 20 minutes?+
Only if the constraints are tiny (n under 15 or so). Otherwise you'll time out. Write a brute force solution for the example to validate your logic, then refactor to DP or math. Don't fall in love with brute force on the real OA.
How do I prepare in 48 hours?+
Memorize the DP template for counting problems: state definition, base cases, recurrence, and order of filling. Practice one binomial or combinatorial problem so you can spot a closed form. Most important: read the Xperi problem three times before you code. The constraints and examples will tell you whether to DP or derive.