leetcode 51. N-Queens

之前写的8皇后蠢爆了，学习了一下，从新写，对递归认识进步一点。

class Solution:
    def solveNQueens(self, n):
        """
        :type n: int
        :rtype: List[List[str]]
        """
        result = []
        board = [-1 for _ in range(n)]
        col, pie, na = set(), set(), set()

        def draw_the_board():
            result.append(['.' * i + 'Q' + '.' * (n - i - 1) for i in board])

        def place_queen(row):
            if row == n:
                draw_the_board()
                return
            for i in range(n):
                if i not in col and i + row not in pie and i - row not in na:
                    board[row] = i
                    col.add(i)
                    pie.add(i + row)
                    na.add(i - row)

                    place_queen(row + 1)

                    col.remove(board[row])
                    pie.remove(board[row] + row)
                    na.remove(board[row] - row)
                    board[row] = -1
        place_queen(0)
        return result