NQueen with Optimizations that traverses only half of the tree

NQueen-optimized.py

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

        def solve_helper(n, slate, i):
            nonlocal result_list
            if i >= n:
                result_list.append(slate[:])
            else:
                for j in filter(lambda x: not is_attack(slate, x), range(n)):
                    slate.append(j)
                    solve_helper(n, slate, i+1)
                    slate.pop()

        def is_attack(slate, n):
            return any([p == n or abs(p - n) == (len(slate)-i) for i, p in enumerate(slate)])

        def mirror(n, soln):
            return [(n-i-1) for i in soln]

        def to_string(soln, n):
            return ["." * soln[i] + "Q" + "." * (n-soln[i]-1) for i in range(n)]

        result_list = []
        for i in range(n//2):
            solve_helper(n, [i], 1)
       
        mirror_list = [mirror(n, i) for i in result_list]

        if n % 2 == 1:
            solve_helper(n, [n//2], 1)

        return [to_string(r, n) for r in (result_list + mirror_list)]