├── .gitignore
├── README.md
├── main.py
└── n_queens.py


/.gitignore:
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1 | .idea
2 | __pycache__
3 | venv
4 | 


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/README.md:
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 1 | # N-Queens
 2 | 
 3 | Solving N-Queens problem by Backtracking (DFS+BFS) in Python.
 4 | 
 5 | [Eight queens puzzle](https://en.wikipedia.org/wiki/Eight_queens_puzzle)
 6 | 
 7 | ## Related
 8 | 
 9 | Another implementaiotn with time/bord plotting, state tracking and benchmarks:
10 | 
11 | [https://github.com/jaya-shankar/n_queens](https://github.com/jaya-shankar/n_queens)
12 | 
13 | Thanks to Jaya Shankar!
14 | 


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/main.py:
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 1 | from n_queens import NQueens
 2 | 
 3 | 
 4 | def main():
 5 |     print('.: N-Queens Problem :.')
 6 |     size = int(input('Please enter the size of board: '))
 7 |     print_solutions = input('Do you want the solutions to be printed (Y/N): ').lower() == 'y'
 8 |     n_queens = NQueens(size)
 9 |     dfs_solutions = n_queens.solve_dfs()
10 |     bfs_solutions = n_queens.solve_bfs()
11 |     if print_solutions:
12 |         for i, solution in enumerate(dfs_solutions):
13 |             print('DFS Solution %d:' % (i + 1))
14 |             n_queens.print(solution)
15 |         for i, solution in enumerate(bfs_solutions):
16 |             print('BFS Solution %d:' % (i + 1))
17 |             n_queens.print(solution)
18 |     print('Total DFS solutions: %d' % len(dfs_solutions))
19 |     print('Total BFS solutions: %d' % len(bfs_solutions))
20 | 
21 | 
22 | if __name__ == '__main__':
23 |     main()
24 | 


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/n_queens.py:
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 1 | from queue import Queue
 2 | 
 3 | 
 4 | class NQueens:
 5 | 
 6 |     def __init__(self, size):
 7 |         self.size = size
 8 | 
 9 |     def solve_dfs(self):
10 |         if self.size < 1:
11 |             return []
12 |         solutions = []
13 |         stack = [[]]
14 |         while stack:
15 |             solution = stack.pop()
16 |             if self.conflict(solution):
17 |                 continue
18 |             row = len(solution)
19 |             if row == self.size:
20 |                 solutions.append(solution)
21 |                 continue
22 |             for col in range(self.size):
23 |                 queen = (row, col)
24 |                 queens = solution.copy()
25 |                 queens.append(queen)
26 |                 stack.append(queens)
27 |         return solutions
28 | 
29 |     def solve_bfs(self):
30 |         if self.size < 1:
31 |             return []
32 |         solutions = []
33 |         queue = Queue()
34 |         queue.put([])
35 |         while not queue.empty():
36 |             solution = queue.get()
37 |             if self.conflict(solution):
38 |                 continue
39 |             row = len(solution)
40 |             if row == self.size:
41 |                 solutions.append(solution)
42 |                 continue
43 |             for col in range(self.size):
44 |                 queen = (row, col)
45 |                 queens = solution.copy()
46 |                 queens.append(queen)
47 |                 queue.put(queens)
48 |         return solutions
49 | 
50 |     def conflict(self, queens):
51 |         for i in range(1, len(queens)):
52 |             for j in range(0, i):
53 |                 a, b = queens[i]
54 |                 c, d = queens[j]
55 |                 if a == c or b == d or abs(a - c) == abs(b - d):
56 |                     return True
57 |         return False
58 | 
59 |     def print(self, queens):
60 |         for i in range(self.size):
61 |             print(' ---' * self.size)
62 |             for j in range(self.size):
63 |                 p = 'Q' if (i, j) in queens else ' '
64 |                 print('| %s ' % p, end='')
65 |             print('|')
66 |         print(' ---' * self.size)
67 | 


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