Python/backtracking/n_queens.py

'''

 The nqueens problem is of placing N queens on a N * N 
 chess board such that no queen can attack any other queens placed
 on that chess board.
 This means that one queen cannot have any other queen on its horizontal, vertical and 
 diagonal lines.

'''
solution = []

def isSafe(board, row, column):
    '''
    This function returns a boolean value True if it is safe to place a queen there considering 
    the current state of the board.

    Parameters :
    board(2D matrix) : board
    row ,column : coordinates of the cell on a board

    Returns :
    Boolean Value

    '''
    for i in range(len(board)):
        if board[row][i] == 1:
            return False
    for i in range(len(board)):
        if board[i][column] == 1:
            return False
    for i,j in zip(range(row,-1,-1),range(column,-1,-1)):
        if board[i][j] == 1:
            return False
    for i,j in zip(range(row,-1,-1),range(column,len(board))):
        if board[i][j] == 1:
            return False
    return True

def solve(board, row):
    '''
    It creates a state space tree and calls the safe function untill it receives a 
    False Boolean and terminates that brach and backtracks to the next 
    poosible solution branch.
    '''
    if row >= len(board):
        '''
        If the row number exceeds N we have board with a successful combination 
        and that combination is appended to the solution list and the board is printed.

        '''
        solution.append(board)
        printboard(board)
        print()
        return 
    for i in range(len(board)):
        '''
        For every row it iterates through each column to check if it is feesible to place a 
        queen there.
        If all the combinations for that particaular branch are successfull the board is 
        reinitialized for the next possible combination.
        '''
        if isSafe(board,row,i):
            board[row][i] = 1
            solve(board,row+1)
            board[row][i] = 0
    return False

def printboard(board):
    '''
    Prints the boards that have a successfull combination.
    '''
    for i in range(len(board)):
        for j in range(len(board)):
            if board[i][j] == 1:
                print("Q", end = " ")
            else :
                print(".", end = " ")
        print()

#n=int(input("The no. of queens")) 
n = 8
board = [[0 for i in range(n)]for j in range(n)]
solve(board, 0)
print("The total no. of solutions are :", len(solution))