mediumLeetCode#reverse-graph-search#dfs

Pacific Atlantic Water Flow

Return cells from which water can flow downhill or level to both the Pacific and Atlantic borders.

The Runtime Theory TeamSeptember 25, 202625 min read

Solve it

Solving happens on the judge — come back and mark it done

Sample cases

inheights=[[1,2,2,3,5],[3,2,3,4,4],[2,4,5,3,1],[6,7,1,4,5],[5,1,1,2,4]]

outintersection includes [0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]

inheights=[[1]]

out[[0,0]]

inheights=[[1,2],[4,3]]

outall four cells

Return cells from which water can flow downhill or level to both the Pacific and Atlantic borders.

A good solution should

State the invariant or decision rule that makes the approach correct.
Explain time and space costs in terms of input size and required output.
Handle empty, minimal, duplicate, and boundary-shaped inputs.

Reasoning prompt

Search backward from each ocean, moving to neighbors of equal or greater height, then intersect reachable sets.

Use the linked judge for the canonical challenge. The examples above are compact TRT checks; create additional tests around the boundary most likely to break your invariant.

One dispatch a week

The trace behind each problem, the tradeoff that explains it, and one technical dispatch per week — no noise.

One technical dispatch per week. No noise.

Not started

A good solution should

Reasoning prompt

More practice in this topic

One dispatch a week