Pacific Atlantic Water Flow
01 · Problem
There is an m x n rectangular island that borders both the Pacific Ocean and Atlantic Ocean. The Pacific Ocean touches the island's left and top edges, and the Atlantic Ocean touches the island's right and bottom edges.
The island is partitioned into a grid of square cells. You are given an m x n integer matrix heights where heights[r][c] represents the height above sea level of the cell at coordinate (r, c).
The island receives a lot of rain, and the rain water can flow to neighboring cells directly north, south, east, and west if the neighboring cell's height is less than or equal to the current cell's height. Water can flow from any cell adjacent to an ocean into the ocean.
Return a 2D list of grid coordinates result where result[i] = [ri, ci] denotes that rain water can flow from cell (ri, ci) to both the Pacific and Atlantic oceans.
02 · Examples
heights = [[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]]
[[0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]]
These 7 cells can reach both oceans. For example, cell [2,2] with height 5 can flow left/up to reach the Pacific (top/left edges) and right/down to reach the Atlantic (bottom/right edges).
heights = [[1]]
[[0,0]]
A single cell island borders both oceans, so water from it can flow to both.
03 · Constraints
- 01m == heights.length
- 02n == heights[r].length
- 031 <= m, n <= 200
- 040 <= heights[r][c] <= 100000
04 · Optimal complexity
- Time
- O(m * n)
- Space
- O(m * n)
Practice it alone or rehearse it as an interview.
Practice Mode gives you an editor and test runs, nothing else. AI Interview Mode puts a voice interviewer on the other side, adds a clock, and ends with a scored summary of the round.