input: Write a python function to decode a list of numbers to a string where 1 maps to 'A', 2 maps to 'B', and 3 maps to 'C'.
  assert decode_message([1,2,3,2,2,3,1,3])=="ABBCBBAC"
  assert decode_message([3,3,2,1,1])=="CCBAA"
  assert decode_message([2,2,2,1])=="BBBA"
output: def decode_message(numbers):
      mapping = {1: 'A', 2: 'B', 3: 'C'}
      return ''.join(mapping[num] for num in numbers)