Fix n-queens problem

[infrablue1]

Describe your change:

Fix wrong solution for n-queens problem which in backtrace folder.

Details

There is an issue about n_queens.py. If I run this script, it will print the wrong result which is different with n_queens_math.py. I think it's enough to check left upper diagonal, right upper diagonal and the same column here. Here are the last few lines of the correct output with my changes:

....... Q
... Q ....
Q ......... Q .......... Q ... Q ............ Q ..... Q ...
The total number of solutions are: 92

[MaximSmolskiy]

@cclauss Can we do something about this?

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