Add union and union_by snippets

snippets/union.md

@@ -0,0 +1,17 @@
+---
+title: union
+tags: list,beginner
+---
+
+Returns every element that exists in any of the two lists once.
+
+Create a `set` with all values of `a` and `b` and convert to a `list`.
+
+```py
+def union(a,b):
+  return list(set(a + b))
+```
+
+```py
+union([1, 2, 3], [4, 3, 2]) # [1,2,3,4]
+```