Compute rainwater accumulation on a height grid

## Mountain Rainfall Accumulation You are given an `m x n` 2D integer grid `elevation`, where `elevation[r][c]` is the terrain height at cell `(r, c)`. Assume **1 unit** of rain falls on **each cell** (uniformly). Water from a cell flows **downhill** by repeatedly moving to a neighboring cell with **strictly lower** elevation until it reaches a **local minimum** (a cell that has no strictly lower neighbor). - Neighbors are the 4 orthogonal cells: up, down, left, right (when in bounds). - From a cell, water moves to the neighbor that represents the **steepest descent**. You may implement this as moving to the neighbor with the **minimum elevation** among all strictly lower neighbors. - If there are multiple equally-steep choices (ties), you may break ties **arbitrarily**, so multiple different correct outputs may exist. ### Task Return an `m x n` integer grid `accum` where: - `accum[r][c]` equals the **total number of rain units** that eventually end at cell `(r, c)` (including the unit that fell on `(r, c)` itself if it is a sink). - All non-sink cells should have `0` in `accum` (since they do not collect water permanently). ### Example **Input** ```text [ [3, 3, 1, -1], [2, 3, 3, 1], [0, 1, 3, 3] ] ``` **One valid output** ```text [ [0, 0, 0, 6], [0, 0, 0, 0], [6, 0, 0, 0] ] ``` **Another valid output** (due to tie-breaking) ```text [ [0, 0, 0, 7], [0, 0, 0, 0], [5, 0, 0, 0] ] ``` (There are 12 cells total, so the sum of all values in `accum` should be 12.)