input: Write a python function to find the longest of two strings.
  assert longestString("Hello", "World") == "Hello"
  assert longestString("Python", "Coding") == "Python"
  assert longestString("Dog", "Elephant") == "Elephant"
output: def longestString(str1, str2):
      if len(str1) >= len(str2):
          return str1
      else:
          return str2