Pacific Atlantic Flow

You are given an `m x n` matrix `heights` where `heights[r][c]` is the height of the cell at row `r` and column `c`. Water can flow from a cell to any of its four neighbors (up, down, left, right) if the neighbor's height is **less than or equal to** the current cell's height. The Pacific Ocean touches the **top and left** edges, while the Atlantic Ocean touches the **bottom and right** edges. Write a function `pacific_atlantic(heights: list[list[int]]) -> list[list[int]]` that returns a list of `[r, c]` pairs for **all** cells from which water can flow to **both** the Pacific and Atlantic oceans. The order of the returned cells does not matter. **Constraints:** - `1 <= m, n <= 200` - `0 <= heights[r][c] <= 10^5` You may assume that the input matrix is always valid and non-empty.

>>> pacific_atlantic([[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]]

>>> pacific_atlantic([[1]])
[[0,0]]