input: Write a function to remove duplicates from an array and return the resulting array in the order of their first appearance. If the array is empty, return an empty array.
  assert removeDuplicates(['a', 'b', 'a', 'c', 'c', 'd'])==['a', 'b', 'c', 'd']
  assert removeDuplicates(['apple', 'banana', 'apple', 'cherry'])==['apple', 'banana', 'cherry']
  assert removeDuplicates(['x', 'y', 'z'])==['x', 'y', 'z']
  assert removeDuplicates([])==[]
output: def removeDuplicates(arr):
      seen = set()
      result = []
      for item in arr:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result