Zig Zag Sequence Fantogame

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.