n_queens.py

def solveNQueens(n):
    """
    Returns all possible combinations of N queens placement.
    :type n: int
    :rtype: List[List[str]]
    """
    board = [['.'] * n for _ in range(n)]
    solutions = []

    def solve_for(n_q, row, b):
        if n_q == 0:
            solutions.append([''.join(row) for row in b])
        else:
            for i in range(row, n):
                if 'Q' in b[i]:
                    continue
                solution_at_row = False
                for j in range(0, n):
                    if is_attacked(i, j, n, b):
                        continue
                    b[i][j] = 'Q'
                    solve_for(n_q - 1, i, b)
                    b[i][j] = '.'
                if not solution_at_row:
                    break
    solve_for(n, 0, board)
    return solutions


def is_attacked(i, j, n, board):
    print(i, j, n, board)
    for k in range(0, n):
        if board[k][j] == 'Q':
            return True
    for m in range(0, n):
        if board[i][m] == 'Q':
            return True

    # parallel to main diagonal
    d, e = (i - j, 0) if i >= j else (0, j - i)
    while d < n and e < n:
        if board[d][e] == 'Q':
            return True
        d += 1
        e += 1

    # parallel to the secondary diagonal
    d = i
    e = j
    while d < n and e >= 0:
        if board[d][e] == 'Q':
            return True
        d += 1
        e -= 1

    d = i
    e = j
    while d >= 0 and e < n:
        if board[d][e] == 'Q':
            return True
        d -= 1
        e += 1
    return False