input: Write a python function to combine two strings such that the lexicographically smaller string is placed first.
  assert combine_strings("apple", "banana") == "applebanana"
  assert combine_strings("zebra", "aardvark") == "aardvarkzebra"
  assert combine_strings("cherry", "cherry") == "cherrycherry"
output: def combine_strings(str1, str2):
      if str1 < str2:
          return str1 + str2
      else:
          return str2 + str1