input: Write a python function to find the character that appears the most frequently in a given string.
  assert max_char('abrakadabra') == 'a'
  assert max_char('hello') == 'l'
  assert max_char('test') == 't'
  assert max_char('aabbcc') == 'a'
output: def max_char(s):
      from collections import Counter
      count = Counter(s)
      return max(count, key=count.get)