input: Write a python function to find the longest string in a list.
  assert find_longest_string(["Apple", "Banana", "Orange", "Mango", "Pineapple"]) == "Pineapple"
  assert find_longest_string(["Red", "Green", "Blue"]) == "Green"
  assert find_longest_string(["Cat", "Dog", "Elephant"]) == "Elephant"
output: def find_longest_string(lst):
      if not lst:
          return None
      return max(lst, key=len)