Find all cells reachable by downhill flow

## Problem You are given an `m x n` grid `heights`, where `heights[r][c]` is the elevation of cell `(r, c)`. A drop of water is placed at a starting cell `start = (sr, sc)`. From a cell, water may flow to any of its **4-directional neighbors** (up/down/left/right) **only if the neighbor has strictly lower height** than the current cell. A cell is considered **wet** if water can reach it by repeatedly flowing according to the rule above (including the start cell). ### Task Return the list (or set) of all wet cells. ### Input - `heights`: 2D integer array of size `m x n` - `start`: two integers `(sr, sc)` with `0 <= sr < m`, `0 <= sc < n` ### Output - All coordinates `(r, c)` that become wet. ### Notes / Constraints - You may return cells in any order. - Assume `m, n` can be up to a few thousand (so your approach should be near-linear in grid size in the worst case). - Heights can be any integers (not necessarily unique).