Problem 15: Lattice paths
Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.
RRDD
RDRD
RDDR
DRRD
DRDR
DDRR
How many such routes are there through a 20×20 grid?
JavaScript
See the Pen Problem 15 by Joshua Bee (@JoshuaBee) on CodePen.