input: Write a function to count the occurrence of each character in a string.
  assert characterCount("Hello World!") == {'H': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}
  assert characterCount("test") == {'t': 2, 'e': 1, 's': 1}
  assert characterCount("Python programming") == {'P': 1, 'y': 1, 't': 1, 'h': 1, 'o': 2, 'n': 2, ' ': 1, 'p': 1, 'r': 2, 'g': 2, 'a': 1, 'm': 2, 'i': 1}
output: def characterCount(s):
      count = {}
      for char in s:
          if char in count:
              count[char] += 1
          else:
              count[char] = 1
      return count