def is_valid(queens, row, col):for i in range(row):if queens[i] == col or abs(queens[i] - col) == abs(i - row):return Falsereturn Truedef solve_n_queens(n, row, queens, result):if row == n:result.append(queens[:])  # 将当前解添加到结果中returnfor col in range(n):if is_valid(queens, row, col):queens[row] = colsolve_n_queens(n, row + 1, queens, result)queens[row] = -1  # 回溯def n_queens(n):result = []queens = [-1] * n  # 初始化皇后位置为-1solve_n_queens(n, 0, queens, result)return result# 测试
n = 8  # 8皇后问题
solutions = n_queens(n)
for solution in solutions:for col in solution:row_str = '.' * col + 'Q' + '.' * (n - col - 1)print(row_str)print()

row_str = ‘.’ * col + ‘Q’ + ‘.’ * (n - col - 1) 这行代码用于构建一个字符串，表示当前行的皇