A knight's tour is a sequence of moves of a knight on a chessboard such that 
the knight visits every square exactly once. If the knight ends on a square 
that is one knight's move from the beginning square (so that it could tour 
the board again immediately, following the same path), the tour is closed 
(or re-entrant); otherwise, it is open.

For more information: https://en.wikipedia.org/wiki/Knight%27s_tour
