05.k

/ Speed test
974~b:|/a:2/+"BR"'0:`5.txt
646~-1+*(2=-':)#^a              / second cut, below
646~*a_1+c+!b-c:&/a             / slower
646~@[(1+b)#0b;(!&/a),a;1b]?0   / faster
646~@[,/(&/a;1+b)#'10b;a;1b]?0  / fastest (at this size) without using tricks here https://www.reddit.com/r/adventofcode/comments/k71h6r/2020_day_05_solutions/geo939w/?context=3
646~@[&(c;1+b-c:&/a);a;0]?1     / approx fastest (notice uses int list instead of boolean list)
646~&/a_1+a                     / Leah (shortest by far!)

/ Second cut - based on insight from: https://www.reddit.com/r/adventofcode/comments/k71h6r/2020_day_05_solutions/geoawah/
974~|/a:2/+"BR"'0:`5.txt
646~-1+*(2=-':)#^a

/ First cut
f:8/2/''"BR"=+0 7^/:
974~|/a:f@0:`5.txt
646~-1+*(2=-':)#^a