input: Write a function to remove all consecutive duplicate characters from a given string. If the string is empty, return an empty string.
  def remove_duplicates(string):
      result = ""
      prev = ''
  
      for char in string:
          if char != prev:
              result += char
              prev = char
      return result
  
  assert remove_duplicates("AAABBCCCD")=="ABCD"
  assert remove_duplicates("A")=="A"
  assert remove_duplicates("ABBA")=="ABA"
  assert remove_duplicates("")==""
  assert remove_duplicates("AAAA")=="A"
output: def remove_duplicates(string):
      result = ""
      prev = ''
  
      for char in string:
          if char != prev:
              result += char
              prev = char
      return result