Programming in Lua

class Solution:
    def solveNQueens(self, n: int) -> List[List[str]]:
        solutions = []
        def isplaceok(board, N, col):
            for i in range(N): # for each queen already placed
                if (board[i] == col) or (board[i] - i == col - N) or (board[i] + i == col + N):
                    return False # place can be attacked
            return True # no attacks; place is ok

        def printsolution(board):
            solutions.append(board[:]) #shallow copy
            for i in range(n):
                for j in range(n):
                    print("X" if board[i] == j else "-", end="")
                print("")
            print("\n")

        def addqueen(board, N):
            if N >= n: # all queens have been placed
                printsolution(board)
            else: # try place the Nth queen
                for col in range(n):
                    if isplaceok(board, N, col):
                        print(board)
                        board[N] = col # place nth queen at column c
                        addqueen(board, N+1)

        addqueen([-1]*n, 0)
        print(solutions)
        return [
            ["." * c + "Q" + "." * (n - c - 1) for c in sol]
            for sol in solutions
        ]