How do you sort an array of only 0s, 1s, and 2s in one pass?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Dutch National Flag uses low, mid, and high pointers: swap 0s to the left, 2s to the right, and advance on 1s.
WHY — Two Pointers instead of guessing?
Why interviewers care about Two Pointers:
question about Two Pointers.
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:
- 1Dutch National Flag uses
low, mid, and high pointers: swap 0s to the left, 2s to the right, and advance on 1s.
- 2One O(n) pass and
O(1) space, no counting sort required.
- 3Counting frequencies then rewriting
is also O(n) but two passes, which is acceptable if you name it.
- 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
Dutch National Flag uses low, mid, and high pointers: swap 0s to the left, 2s to the right, and advance on 1s. One O(n) pass and O(1) space, no counting sort required.