input: Write a python function to concatenate a list of strings into a single string.
  assert concatenate_list(["Hello", " ", "world", "!"]) == "Hello world!"
  assert concatenate_list(["Python", "is", "fun"]) == "Pythonisfun"
  assert concatenate_list(["a", "b", "c", "d"]) == "abcd"
output: def concatenate_list(lst):
      return "".join(lst)