diff --git a/aoc2018.nim b/aoc2018.nim
index ca7239a..7790a81 100644
--- a/aoc2018.nim
+++ b/aoc2018.nim
@@ -1,17 +1,18 @@
+import std/intsets
 import std/sequtils
 import tools/aoc
 
 let test: AOCDay = initAOCDay(1)
 echo test.getInputInt().foldl(a + b)
 
-var seen: seq[int]
+var seen: IntSet = initIntSet()
 var freq: int = 0
 var found: bool = false
 while not found:
   for x in test.getInputInt():
     freq += x
     if freq notin seen:
-      seen.add(freq)
+      seen.incl(freq)
     else:
       echo freq
       found = true