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| 1 | +# -*- coding: utf-8 -*- |
| 2 | +""" |
| 3 | +Created on Wed Oct 7 20:51:57 2020 |
| 4 | + |
| 5 | +@author: anjalisingh |
| 6 | +""" |
| 7 | + |
| 8 | +class Solution(object): |
| 9 | + def solveNQueens(self, n): |
| 10 | + # i is column index |
| 11 | + # left: left diagonal: \ level-i |
| 12 | + # right: right diagonal: / level+i |
| 13 | + self.col = [0]*n # not occupied column |
| 14 | + self.left = [0]*(2*n-1) # number of left diagonal |
| 15 | + self.right = [0]*(2*n-1) |
| 16 | + board = [['.' for x in range(n)] for y in range(n)] |
| 17 | + self.resultBoard = [] |
| 18 | + self.backTrack(n, 0, board) |
| 19 | + return self.resultBoard |
| 20 | + |
| 21 | + def backTrack(self, n, level, board): |
| 22 | + if level == n: # finish |
| 23 | + res = [] |
| 24 | + for i in range(n): |
| 25 | + res.append(''.join(board[i])) |
| 26 | + self.resultBoard.append(res) |
| 27 | + return |
| 28 | + for i in range(n): # iterate every column |
| 29 | + # if col, left, right are all not accupied, put a queue here |
| 30 | + if not self.col[i] and not self.left[level-i] and not self.right[level+i]: |
| 31 | + board[level][i] = 'Q' |
| 32 | + self.col[i] = 1 |
| 33 | + self.left[level-i] = 1 |
| 34 | + self.right[level+i] = 1 # choose |
| 35 | + self.backTrack(n, level+1, board) # explore |
| 36 | + board[level][i] = '.' # un choose |
| 37 | + self.col[i] = 0 |
| 38 | + self.left[level-i] = 0 |
| 39 | + self.right[level+i] = 0 |
| 40 | + |
| 41 | +# Queen = Solution() |
| 42 | +# print(Queen.solveNQueens(4)) |
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