diff --git a/snippets/difference.md b/snippets/difference.md
index 949a26e75..0ae57c7c4 100644
--- a/snippets/difference.md
+++ b/snippets/difference.md
@@ -2,11 +2,12 @@
 
 Returns the difference between two iterables.
 
-Use list comprehension to only keep values not contained in `b`
+Use list comprehension to only keep values not contained in `b`. Use set to test `in` in `O(1)` time.
 
 ```python
 def difference(a, b):
-    return [item for item in a if item not in b]
+    _b = set(b)
+    return [item for item in a if item not in _b]
 ```
 ``` python
 difference([1, 2, 3], [1, 2, 4]) # [3]