Pacific Atlantic Water Flow
Given an m x n matrix of heights Find all cells where water can flow to BOTH Pacific and Atlantic oceans Pacific touches left and top edges, Atlantic touches right and bottom Water flows from cell to adjacent cells with height <= current height Example: [[1,2,3],[4,5,6],[7,8,9]] => [[0,2],[1,2],[2,0],[2,1],[2,2]]
Pacific Atlantic Water Flow is a traversal problem: the hard part is modeling the state you visit. Interviewers love it because it reveals your traversal modeling and your edge-case discipline. Be clear about what a state is; that's the difference between BFS/DFS working or blowing up.
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