为了面试复习一下经典的题目

八皇后

当然有更短更精奇的写法，但是我觉得我这个比较容易理解吧？。。。

 1 count = 0
 2 
 3 
 4 def eight_queens(queens_position, depth):
 5     """
 6     Computer how many solutions are there for eight queens problem
 7     """
 8 
 9     def is_valid(board, pos):
10         """
11         Check if given position is valid
12         """
13 
14         def isOnSameLine(pos1, pos2):
15             """
16             check if this two position is on diagonal or vertical line
17             """
18             if pos1[1] == pos2[1]:
19                 return True
20 
21             # Slope is 1
22             # y = (x - a) + b
23             if pos2[1] == (pos2[0] - pos1[0] + pos1[1]):
24                 return True
25 
26             # Slope is -1
27             # y = -(x - a) + b
28             if pos2[1] == (-(pos2[0] - pos1[0]) + pos1[1]):
29                 return True
30 
31             return False
32 
33         for lop in range(len(board)):
34             if isOnSameLine((lop, board[lop]), pos):
35                 return False
36         return True
37 
38     if depth == 8:
39         print(queens_position)
40         global count
41         count += 1
42         return
43 
44     # Trying to place queen
45     for lop in range(8):
46         if is_valid(queens_position, (len(queens_position), lop)):
47             tmp_list = list(queens_position)
48             tmp_list.append(lop)
49             eight_queens(tmp_list, depth + 1)
50 
51 
52 if __name__ == '__main__':
53     eight_queens([], 0)
54     print(count)