How do you compute trapped rain water given elevation heights?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Two pointers with left_max and right_max: the water at the shorter side is bounded by that side's max, so you advance the shorter pointer.
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:
- 1Two pointers with left_max
and right_max: the water at the shorter side is bounded by that side's max, so you advance the shorter pointer.
- 2Time is O(n) and
space is O(1).
- 3Prefix/suffix max arrays are
O(n) space and easier to explain first.
- 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
Two pointers with left_max and right_max: the water at the shorter side is bounded by that side's max, so you advance the shorter pointer. Time is O(n) and space is O(1).