input: Write a python function to find the highest occurring character in a string.
  assert highest_occurring_char("test string") == "t"
  assert highest_occurring_char("hello world") == "l"
  assert highest_occurring_char("aabbccdd") == "a"
output: def highest_occurring_char(s: str) -> str:
      from collections import Counter
      count = Counter(s)
      max_count = max(count.values())
      for char, freq in count.items():
          if freq == max_count:
              return char