Leetcode: N-Queens

Question

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens’ placement, where ‘Q’ and ‘.’ both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[
[“.Q..”, // Solution 1
“…Q”,
“Q…”,
“..Q.”],

[“..Q.”, // Solution 2
“Q…”,
“…Q”,
“.Q..”]
]
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Solution

Get idea from here1, here2

class Solution(object):
    def solveNQueens(self, n):
        """ :type n: int :rtype: List[List[str]] """

        res = []
        columnInRow = [0]*n
        self.helper( n, 0, columnInRow , res)
        return res

    def helper(self, n, row, columnInRow, res):
        if row==n:
            item = []
            for indr in range(n):
                row = ""
                for indc in range(n):
                    if indc==columnInRow[indr]:
                        row += 'Q'
                    else:
                        row += '.'
                item.append(row)
            res.append(item)
            return

        for ind in range(n):
            columnInRow[row] = ind
            if self.check(row,columnInRow):
                self.helper(n, row+1, columnInRow, res)

    def check(self, row, columnInRow):
        for ind in range(row):
            if columnInRow[ind]==columnInRow[row] or abs(columnInRow[ind]-columnInRow[row])==(row-ind):
                return False
        return True