How do you find an element that appears exactly once in an array?
This question tests bitwise operations or hash map usage for frequency counting. It distinguishes candidates who know low-level optimizations.
Why Interviewers Ask This
Interviewers look for proficiency in bitwise XOR operations which offer O(n) time and O(1) space solutions. It shows if you can think beyond standard library functions to write highly efficient code.
How to Answer This Question
First, consider using a hash map to count frequencies, noting the space trade-off. Then, pivot to the XOR approach where identical numbers cancel out. Explain why XOR works mathematically and walk through a small example. Conclude by comparing the two methods regarding memory efficiency.
Key Points to Cover
- Utilize XOR property
- Achieve O(n) time complexity
- Maintain O(1) space complexity
- Compare alternative solutions
Sample Answer
The most efficient method uses the XOR operation because a XOR a equals zero. By XORing all elements, duplicates disappear leaving only the unique number. This runs in linear time without extra space. A hash map approach…
Common Mistakes to Avoid
- Using nested loops for O(n^2)
- Ignoring the uniqueness constraint
- Forgetting edge cases with empty arrays
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