Minimum Flips to Alternate Binary String (With K Flip Window)
Reported by candidates from Google's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Google sent you a string problem in May 2024, and it's about flipping bits to make an alternating pattern. You have maybe 48 hours. The trick here is that you're not flipping individual bits one at a time. There's a window constraint, a limited set of flips, and you need to find the minimum number of operations to make the string alternate between 0s and 1s. StealthCoder will have your back if the sliding window logic gets fuzzy during the live OA.
Pattern and pitfall
This is a sliding window problem masquerading as a string manipulation problem. The core insight: there are only two valid alternating patterns (0101... or 1010...), so you calculate the cost to transform each section of the string into both patterns, then pick the cheaper one. The window constraint limits consecutive flips, which means you can't just flip any arbitrary substring. You'll likely need to track the cost of each flip operation and use a greedy or two-pointer approach to ensure no more than K flips happen in any window. The common miss is not recognizing that you can precompute both target patterns and compare them in O(n) time. StealthCoder reads the problem faster than you can parse it.
Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.
You can drill Minimum Flips to Alternate Binary String (With K Flip Window) 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 by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge.
Get StealthCoderRelated leaked OAs
This OA pattern shows up on LeetCode as minimum window substring. If you have time before the OA, drill that.
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Google reuses patterns across OAs. Made by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Minimum Flips to Alternate Binary String (With K Flip Window) FAQ
What are the two alternating patterns I should compare?+
0101... and 1010.... That's it. Calculate the flip cost for the entire string to match each pattern, then pick the minimum. Both patterns are valid alternating strings, so you must test both.
How do I handle the K flip window constraint?+
The window limits how many flips can occur in any consecutive substring. Track flips as you scan left to right. If you exceed K flips in a sliding window, you've hit the boundary. This often requires a two-pointer or greedy pass.
Is this a DP problem or just greedy with two pointers?+
It's greedy with a sliding window. You don't need DP for the pattern matching part. DP bloat is a common trap. Just track flip counts as you slide and compare both alternating target patterns.
What's the time complexity I should aim for?+
O(n) is the goal. Two full passes over the string (one for each target pattern) plus a single window validation pass. Anything worse than O(n log n) suggests you're overcomplicating it.
How do I prepare for this in 24 hours?+
Understand alternating binary strings and how to count mismatches in O(n) time. Practice sliding window basics. The pattern is string plus window constraint, not some exotic algorithm. Confidence comes from recognizing the template.