input: Write a function to find the frequency of each character in a given string.
  assert find_character_frequency("abbccccdddde") == {'a': 1, 'b': 2, 'c': 4, 'd': 4, 'e': 1}
  assert find_character_frequency("hello") == {'h': 1, 'e': 1, 'l': 2, 'o': 1}
  assert find_character_frequency("test") == {'t': 2, 'e': 1, 's': 1}
  assert find_character_frequency("aabbcc") == {'a': 2, 'b': 2, 'c': 2}
output: def find_character_frequency(s):
      frequency = {}
      for char in s:
          if char in frequency:
              frequency[char] += 1
          else:
              frequency[char] = 1
      return frequency