Get Substring
Reported by candidates from Wells Fargo's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Wells Fargo sent you a substring problem in October, and you're staring at it now. This is a classic sliding-window setup, and candidates either nail it or overthink it. The trick is recognizing that you're tracking a window of characters and moving both pointers in sync, not nested loops. If you freeze during the OA, StealthCoder reads the prompt and feeds you the window skeleton in real time so you don't lose momentum.
Pattern and pitfall
Substring problems live in sliding-window territory. You maintain a left and right pointer, expand the window by moving right, and shrink it from the left when a constraint breaks. The pitfall is nested loops, which tank performance. Wells Fargo expects O(n) time, not O(n squared). Track what's in your current window using a hash map or set, depending on whether duplicates matter. If the problem asks for longest, shortest, or the substring itself, the pattern stays the same: expand, check validity, contract. StealthCoder gives you the window template if you blank, so you can code confidently even if the exact substring logic escapes you mid-assessment.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Get Substring 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. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as longest substring without repeating characters. If you have time before the OA, drill that.
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Make sure you actually pass Wells Fargo's OA.
Wells Fargo reuses patterns across OAs. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Get Substring FAQ
Is this just LeetCode 3, or is Wells Fargo asking something different?+
Could be, could be a variant. The core pattern is identical: two pointers, expand right, shrink left when broken. If it's longest substring without repeating, that's LeetCode 3. If it's about finding a target substring or matching a pattern, the window still moves the same way. Adapt the validity check, not the skeleton.
What's the most common mistake on sliding-window substring problems?+
Nested loops. Candidates revert to brute force and check every substring. That's O(n squared) and fails on large inputs. Two pointers, one pass. Also forgetting to update your result before shrinking the window. Check validity, store answer, then contract.
How do I track what's in the window efficiently?+
Use a dictionary or hash map to count characters, or a set if you only care whether something exists. When you expand, add the new character. When you shrink, decrement or remove. Checking membership is O(1). Don't iterate through the window every step.
Do I need to handle edge cases like empty strings or single characters?+
Yes. Empty input should return empty or 0 depending on what the problem asks. Single character is usually valid. Check the problem statement for what to return if no valid substring exists. Handle it before you write the main loop.
Can I solve this in one pass or do I need two?+
Sliding-window is one pass. Left pointer starts at zero, right pointer expands to the end. You never reset the window position to the beginning. That's the whole point. If you're resetting or iterating twice, you're not sliding.