# This program solves the Eight Queens problem.
# This version prints a single solution.

from ChessBoard import *

# Places queens on the given board,
# starting with the given column.
# Returns True if successful and False if no solution found.
def place_queens(board, col = 0):
    if not board.valid():
        return False
    elif col >= board.size():
        return True
    else:
        for row in range(board.size()):
            board.place(row, col)              # choose
            if place_queens(board, col + 1):   # explore
                return True
            board.remove(row, col)             # un-choose
        return False

def main():
    size = int(input("Board size? "))
    board = ChessBoard(size)
    if place_queens(board):
        print("One solution is as follows:")
        print(board)
    else:
        print("No solution found.")

main()