Build a quadtree

A quadtree splits a 2D region into four quadrants, recursively: every node is either a leaf or has exactly four children.

Build one yourself. One version builds it from a square grid, splitting until a region holds a single value (values are not limited to 0 and 1). Another stores 2D geographic points. Ask the questions that shape the build, such as whether the side length is a power of two, and give the cost.