input: def removeDuplicates(arr: List[str]) -> List[str]:
      """Remove all duplicate elements from a given array of strings.
      >>> removeDuplicates(['a', 'b', 'a', 'c', 'c', 'd'])
      ['a', 'b', 'c', 'd']
      >>> removeDuplicates(['apple', 'banana', 'apple', 'orange', 'banana', 'grape'])
      ['apple', 'banana', 'orange', 'grape']
      """
output:     seen = set()
      result = []
      for item in arr:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result