input: Write a python function to find the number of common elements in two lists.
  assert intersection([1, 2, 3, 4], [3, 4, 5, 6]) == 2
  assert intersection([10, 20, 30], [30, 40, 50]) == 1
  assert intersection([1, 2, 3], [4, 5, 6]) == 0
output: def intersection(lst1, lst2):
      return len(set(lst1) & set(lst2))