Do day 13 and add cleaner solution to day 18.

README.md

@@ -2,7 +2,7 @@
 
 - Day 1: 40:00 (I don't know what I am doing.)
 - Day 2: 14:15 (Okay, but far way from leaderboard.)
-- Day 3: 1st 20:00, 2nd 70:00... (I had a logic error that took me a while to find.) 
+- Day 3: 1st 20:00, 2nd 70:00... (I had a logic error that took me a while to find.)
 - Day 4: 1st 9:06, 2nd 22:31; it wasn't hard but I didn't think quick enough :/
 - Day 5: 1st 25:00, 2nd 1:55:00; Required patience and accuracy
 - Day 6: 13:54; I was slow because I thought it is much harder?
@@ -12,7 +12,9 @@
 - Day 10: 180:00; this one was hard for me.
 - Day 11: 68:00; okay but not elegant and way too slow ofc; x-ray solution would have been neat
 - Day 12: 52:00 and 22:00 for leaderboard; had the right idea and I am good at this type of problem
-- ...
+- Day 13: 90:00; pretty straightforward  but way too slow
+- Day 14:
+- Day 15:
 - Day 16: 00:27:30 745; best placement so far, of course still horribly slow
 - Day 17: a couple of hours; I realized that I need A* after a while; reused
   implementation from Project Euler but improved with heapq which was super fun