input: Write a function to find the index of the first unique character in a string.
  assert find_first_unique_char_index("leetcode")==0
  assert find_first_unique_char_index("loveleetcode")==2
  assert find_first_unique_char_index("aabb")==-1
output: def find_first_unique_char_index(s: str) -> int:
      char_count = {}
      for char in s:
          char_count[char] = char_count.get(char, 0) + 1
      for index, char in enumerate(s):
          if char_count[char] == 1:
              return index
      return -1