How do you solve N-Queens, placing n queens so none attack?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Backtrack row by row, skipping used columns and both diagonals (r-c and r+c bit sets or boolean arrays).
WHY — Recursion instead of guessing?
Why interviewers care about Recursion:
question about Recursion.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Backtrack row by row,
skipping used columns and both diagonals (r-c and r+c bit sets or boolean arrays).
- 2Worst-case time is exponential,
roughly O(n!) with pruning, and space is O(n) for the board state.
- 3Bitmasks of columns and
diagonals speed the inner check to O(1).
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Backtrack row by row, skipping used columns and both diagonals (r-c and r+c bit sets or boolean arrays). Worst-case time is exponential, roughly O(n!) with pruning, and space is O(n) for the board state.