  The 15 puzzle is a classic reconfiguration puzzle with fifteen uniquely
labeled unit squares within a $4 \times 4$ board in which the goal is to slide
the squares (without ever overlapping) into a target configuration. By
generalizing the puzzle to an $n \times n$ board with $n^2-1$ squares, we can
study the computational complexity of problems related to the puzzle; in
particular, we consider the problem of determining whether a given end
configuration can be reached from a given start configuration via at most a
given number of moves. This problem was shown NP-complete in Ratner and Warmuth
(1990). We provide an alternative simpler proof of this fact by reduction from
the rectilinear Steiner tree problem.
