Zig Zag Sequence


Continue the sequence with the correct number.
There is a list of numbers (1 - 9) that will be added in sequence ... can you know what the next number is?
  in the list there are:
in the easy level: between 2 and 4 numbers.
in the medium level: between 3 and 5 numbers.
in the hard level: between 3 and 10 numbers.