Generate all safe queen placements on board

You are given an integer `n` representing the size of a chessboard (`n × n`). You need to place `n` queens on the board so that no two queens attack each other. A queen can attack another piece if they are on the **same row**, **same column**, or **same diagonal**. Return **all distinct valid configurations** of the board. - Each configuration should be represented as an array (or list) of `n` strings. - Each string has length `n` and represents one row of the board. - Character `'Q'` represents a queen, and `'.'` represents an empty square. Two configurations are considered different if they differ in at least one row string. You may assume: - `1 ≤ n ≤ 9`. **Example** Input: ```text n = 4 ``` One valid output (order of configurations and rows does not matter): ```text [ [".Q..", "...Q", "Q...", "..Q."], ["..Q.", "Q...", "...Q", ".Q.."] ] ``` In each configuration above, there are 4 queens on the board, and no two queens attack each other.