Add intersection snippet

snippets/intersection.md

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+---
+title: intersection
+tags: list,beginner
+---
+
+Returns a list of elements that exist in both lists.
+
+Create a `set` from `b`, then use list comprehension on `a` to only keep values contained in both lists.
+
+```py
+def intersection(a, b):
+  _b = set(b)
+  return [item for item in a if item in _b]
+```
+
+```py
+intersection([1, 2, 3], [4, 3, 2]) # [2, 3]
+```