input: Write a function to concatenate a list of strings.
  assert concatenateString(["Hello", "World", "How", "are", "you"]) == "HelloWorldHowareyou"
  assert concatenateString(["This", "is", "a", "test"]) == "Thisisatest"
  assert concatenateString(["Python", "is", "fun"]) == "Pythonisfun"
output: def concatenateString(strings):
      return ''.join(strings)