input: Write a python function to delete duplicate characters from a string.
  assert delete_duplicate("w3python")=="w3python"
  assert delete_duplicate("programming")=="progamin"
  assert delete_duplicate("hello")=="helo"
output: def delete_duplicate(s):
      result = ""
      seen = set()
      for char in s:
          if char not in seen:
              seen.add(char)
              result += char
      return result