1+ #! /usr/bin/python3
2+ import sys
3+
4+ def nqueens (board_width ):
5+ board = [0 ]
6+ current_row = 0
7+ while True :
8+ conflict = False
9+
10+ for review_index in range (0 ,current_row ):
11+ left = board [review_index ]- (current_row - review_index )
12+ right = board [review_index ]+ (current_row - review_index );
13+ if (board [current_row ]== board [review_index ]or (left >= 0 and left == board [current_row ])or (right < board_width and right == board [current_row ])):
14+ conflict = True ;
15+ break
16+
17+ if (current_row == 0 and conflict == False ):
18+ board .append (0 )
19+ current_row = 1
20+ continue
21+
22+ if (conflict == True ):
23+ board [current_row ]+= 1
24+
25+ if (current_row == 0 and board [current_row ]== board_width ):
26+ print ("No solution exists for specificed board size." )
27+ return None
28+
29+ while True :
30+ if (board [current_row ]== board_width ):
31+ board [current_row ]= 0
32+ if (current_row == 0 ):
33+ print ("No solution exists for specificed board size." )
34+ return None
35+
36+ board .pop ()
37+ current_row -= 1
38+ board [current_row ]+= 1
39+
40+ if board [current_row ]!= board_width :
41+ break
42+ else :
43+ current_row += 1
44+ if (current_row == board_width ):
45+ break
46+
47+ board .append (0 )
48+ return board
49+
50+ def print_board (board ):
51+ if (board == None ):
52+ return
53+
54+ board_width = len (board )
55+ for row in range (board_width ):
56+ line_print = []
57+ for column in range (board_width ):
58+ if column == board [row ]:
59+ line_print .append ("Q" )
60+ else :
61+ line_print .append ("." )
62+ print (line_print )
63+
64+
65+ if __name__ == '__main__' :
66+ default_width = 8
67+ for arg in sys .argv :
68+ if (arg .isdecimal ()and int (arg )> 3 ):
69+ default_width = int (arg )
70+ break
71+
72+ if (default_width == 8 ):
73+ print ("Running algorithm with board size of 8. Specify an alternative Chess board size for N-Queens as a command line argument." )
74+
75+ board = nqueens (default_width )
76+ print (board )
77+ print_board (board )