广度优先搜索n皇后问题python代码

以下是广度优先搜索解决n皇后问题的Python代码：

from queue import Queue

def is_valid(board, row, col):
    for i in range(row):
        if board[i] == col or \
        abs(board[i] - col) == abs(i - row):
            return False
    return True

def bfs(n):
    q = Queue()
    q.put([])
    while not q.empty():
        board = q.get()
        if len(board) == n:
            return board
        for i in range(n):
            if is_valid(board, len(board), i):
                q.put(board + [i])

def print_board(board):
    for row in board:
        line = ""
        for col in range(len(board)):
            if col == row:
                line += "Q "
            else:
                line += ". "
        print(line)

n = 8
board = bfs(n)
print_board(board)

该代码使用队列实现广度优先搜索算法，每次取出一个棋盘，检查其是否合法，如果合法就将其所有可能的下一步棋盘加入队列中。当棋盘长度为n时，返回该棋盘。最后打印出解决方案。