Succint NQueen solution

NQueen.py

class Solution:
    def solveNQueens(self, n: int) -> List[List[str]]:

        def solve_helper(n, slate, new_col):
            nonlocal result
            if new_col >= n:
                result.append(slate[:]) # copy the slate, else we will have empty lists in result as elements are removed from slate
            else:
                for new_row in filter(lambda nr: not is_attack(slate, nr, new_col), range(n)): # find the position where the new queen in the new column is not being attacked by existing queens on the board
                    slate.append(new_row)
                    solve_helper(n, slate, new_col+1) # find all possible positions for the queen on the next column
                    slate.pop()

        def is_attack(slate, new_row, new_col): # slate is current configuration of queens, n is the next position of queen in len(slate) column
            return any([row == new_row or abs(new_row - row) == (new_col - col) for col, row in enumerate(slate)])

        def to_string(soln, n): # soln is a board configuration where queens do not attack each other, n is the size of board
            return ["." * soln[i] + "Q" + "." * (n-soln[i]-1) for i in range(n)]
        
        result = []
        solve_helper(n, [], 0)
        return [to_string(r, n) for r in result]