In chess, pieces move on a board of 8 ranks and 8 files. A chess queen can attack along a file a rank or a diagonal. It is possible to place 8 queens on the board, such that no queen is under attack by another one. When we list the rank on which the queen is for each file, one solution looks like this:

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Print all solutions to the N queens problem, one per a line, for boards of sizes 4-8.

External links: Rosetta Code, Wikipedia