  We prove that the 2017 puzzle game ZHED is NP-complete, even with just 1
tiles. Such a puzzle is defined by a set of unit-square 1 tiles in a square
grid, and a target square of the grid. A move consists of selecting an
unselected 1 tile and then filling the next unfilled square in a chosen
direction from that tile (similar to Tipover and Cross Purposes). We prove
NP-completeness of deciding whether the target square can be filled, by a
reduction from rectilinear planar monotone 3SAT.
