The Skyline Problem in Racket

1) Choose highest building. Draw it's roof horizontal only.

2) Repeat 1) choosing building by next height lower, unless lower building roof within range of an existing rendered roof x+width, then exclude that building from rendering.

3) Once all buildings roofs drawn, draw the building side verticals downwards from each until they hit a roof or ground (either from a cache or detecting pixel colour), in which case stop.