func_code: def union(x, y):
      answer = []
      check = {}
      i = 0
      while i < len(x):
          if not x[i] in check:
              answer.append(x[i])
              check[x[i]] = True
          elif not y[i] in check:
              answer.append(y[i])
              check[y[i]] = True
          i = i + 1