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Quick Summary
Filter an input character array by excluding characters found in a separate ignore list.
Verify if the remaining sequence reads identically forwards and backwards.
Optimize space usage by avoiding upfront array copying during the filtering process.
What This Tests
Two-pointer technique for symmetric traversal
Efficient membership testing using hash-based lookups
In-place validation logic with minimal auxiliary space
Handling edge cases like completely filtered or single-character results
Common Patterns
Two Pointers
Hashing
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