input: Write a python function to find the longest string in an array.
  assert find_longest_string(["apple", "banana", "cherry", "date"]) == "banana"
  assert find_longest_string(["dog", "elephant", "cat"]) == "elephant"
  assert find_longest_string(["a", "ab", "abc", "abcd"]) == "abcd"
output: def find_longest_string(arr):
      if not arr:
          return ""
      return max(arr, key=len)