Day 9: Use set instead of list to speed up lookups

9.py

@@ -33,7 +33,7 @@ def basin_size(map, x, y, taken):
     if (x, y) in taken:
         return 0
     size = 1
-    taken.append((x, y))
+    taken.add((x, y))
     if x > 0:
         size += basin_size(map, x - 1, y, taken)
     if y > 0:
@@ -45,7 +45,7 @@ def basin_size(map, x, y, taken):
     return size
 
 basins = []
-taken = []
+taken = set()
 
 for x in range(len(input)):
     for y in range(len(input[0])):