input: Write a python function to count unique characters in an array and return their frequencies.
  assert count_unique_chars(["a", "b", "c", "a", "d", "e", "c"]) == {'c': 2, 'e': 1, 'a': 2, 'b': 1, 'd': 1}
  assert count_unique_chars(["x", "y", "z", "x", "y", "x"]) == {'x': 3, 'y': 2, 'z': 1}
  assert count_unique_chars(["1", "2", "3", "2", "1", "1"]) == {'1': 3, '2': 2, '3': 1}
output: def count_unique_chars(arr):
      freq_dict = {}
      for char in arr:
          if char in freq_dict:
              freq_dict[char] += 1
          else:
              freq_dict[char] = 1
      return freq_dict