Swim in Rising Water

You are given an `n x n` integer matrix `grid` where each value `grid[i][j]` represents the elevation at that point `(i, j)`. The rain starts to fall. At time `t`, the depth of the water everywhere is `t`. You can swim from a square to another 4-directionally adjacent square if and only if the elevation of both squares individually are at most `t`. You can swim infinite distances in zero time. Of course, you must stay within the boundaries of the grid during your swim. Return _the least time until you can reach the bottom right square _`(n - 1, n - 1)`_ if you start at the top left square _`(0, 0)`.   **Example 1:** **Input:** grid = [[0,2],[1,3]] **Output:** 3 Explanation: At time 0, you are in grid location (0, 0). You cannot go anywhere else because 4-directionally adjacent neighbors have a higher elevation than t = 0. You cannot reach point (1, 1) until time 3. When the depth of water is 3, we can swim anywhere inside the grid. ``` **Example 2:** **Input:** grid = [[0,1,2,3,4],[24,23,22,21,5],[12,13,14,15,16],[11,17,18,19,20],[10,9,8,7,6]] **Output:** 16 **Explanation:** The final route is shown. We need to wait until time 16 so that (0, 0) and (4, 4) are connected. ```   **Constraints:** - `n == grid.length` - `n == grid[i].length` - `1 <= n <= 50` - `0 <= grid[i][j] < n2` - Each value `grid[i][j]` is **unique**.