Pacific Atlantic Water Flow

Graphs · reverse traversal · Search backwards from both destinations · Stretch · about 32 min

Why it matters

Reversing a search direction can turn many expensive searches into two simple ones.

How it connects

Instead of asking each cell whether it can reach two oceans, begin at the oceans and reverse the question.

Try first

From an ocean edge, which neighbouring heights can reach back to it?

Interview cue

Reverse traversal turns “every cell reaches two oceans” into two shared reachability searches.