input: Write a python function to count the occurrences of each character in a string.
  assert count_characters("Hello World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert count_characters("banana") == {'b': 1, 'a': 3, 'n': 2}
  assert count_characters("") == {}
output: def count_characters(s):
      char_count = {}
      for char in s:
          if char in char_count:
              char_count[char] += 1
          else:
              char_count[char] = 1
      return char_count