Sum of Compressed Num for All Subarrays
Reported by candidates from Wells Fargo's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Wells Fargo hit you with this in October: a subarray problem that requires you to understand what 'compressed number' means and then sum it across all possible subarrays. The title is deliberately vague, which is the real test. You're not studying this tonight. You're here because you need to know the pattern fast, and if you blank on the definition during the live OA, StealthCoder will see it on screen and feed you the approach.
Pattern and pitfall
Without the full problem text, the likely play is this: 'compressed number' probably means removing consecutive duplicates, converting the result to an integer, and summing that value across every contiguous subarray. The pattern is prefix-sum combined with careful string manipulation or digit extraction. The pitfall is trying to brute-force every subarray separately, which will TLE. You need to precompute compressed values and use a running sum. If the definition is different (like run-length encoding count, or digit compression), the core trick stays the same: recognize the transformation rule once, then aggregate efficiently. StealthCoder will clarify the compressed definition from the problem statement if you freeze.
StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.
You can drill Sum of Compressed Num for All Subarrays 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. If you're reading this with an OA window open, you're who this was built for.
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Sum of Compressed Num for All Subarrays FAQ
What does 'compressed number' actually mean here?+
The input doesn't specify it fully, but based on the title and Wells Fargo's style, it likely means removing consecutive duplicate digits and treating the result as a number. Example: 1223 becomes 123. You must verify this against the problem statement on the OA.
Do I really need to check all subarrays?+
Yes, but not individually. Identify the compression rule, precompute compressed values for each subarray in O(n^2) time, and sum them. If that TLEs, look for a DP or prefix-sum trick to avoid recomputing overlaps.
What's the real gotcha here?+
The vague definition. Once you understand what 'compressed' means from the examples, the solution is straightforward. The gotcha is spending 10 minutes guessing the rule. Read the examples first.
Is this a dynamic programming problem?+
Not necessarily. It's more about efficient aggregation. If the brute-force O(n^2) subarray enumeration fits the constraints, go with that. DP would be a fallback if n is very large.
How do I prepare in 48 hours?+
You can't cram this without seeing the examples. Focus on subarray iteration patterns and understand how to extract and transform digits. If you blank on the definition live, StealthCoder will parse it from the problem text.