nQueens.py : Solve the N-Queens problem

The N-queens problem is: 

  Given an empty NxN chessboard, place N queens on the board so that no queen can attack any other, i.e. such that no two queens share the same row, column or diagonal.

Sample output :-

[<username> ~]$ py nQueens.py 8
_ _ _ _ Q _ _ _
_ _ _ _ _ _ Q _
_ Q _ _ _ _ _ _
_ _ _ _ _ Q _ _
_ _ Q _ _ _ _ _
Q _ _ _ _ _ _ _
_ _ _ Q _ _ _ _
_ _ _ _ _ _ _ Q

Here, Q's indicate the positions of queens on the board.