Zig Zag Sequence

Continue the sequence with the correct number.

There are a list of numbers (1 - 9) that will be sum in sequence... can you know what is the next number?

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.