051 N Queens

import copy
class Solution:
    def __init__(self):
        self.ans = []
    # @param {integer} n
    # @return {integer}
    def solveNQueens(self, n):
        self.helper(0,[["."]* n for i in range(0,n)],n)
        return self.ans

    def helper(self, level, tmp, n):
        if level == n:
            a = []
            for t in tmp:
                a.append("".join(t))
            self.ans.append(a)
            return
        for pos in range(0,n):
            flag = True
            for i in range(0, level):
                if not flag:
                    break
                if tmp[i][pos] == "Q":
                    flag = False
                    break
                for j in range(0, n):
                    if tmp[i][j]== "Q" and (level-i == pos-j or level-i==j-pos):
                        flag = False
                        break
            if flag:
                tmp[level][pos] = "Q"
                self.helper(level+1, tmp, n)
                tmp[level][pos] = "."