You are given two integers m and n.

Construct and return an m $\times$ n grid represented as a list of strings, where each character is either '.' denoting an empty cell or '#' denoting an obstacle, such that there is exactly $1$ valid path from the top left cell $(0, 0)$ to the bottom right cell (m $- 1$, n $- 1$).

A valid path starts at $(0, 0)$, ends at (m $- 1$, n $- 1$), and moves only right or down through empty cells.

If multiple valid grids exist, return any of them.

Constraints:

• $1 \le$ m, n $\le 25$

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Implement your solution in the following coding playground.