SPOILERS!
I think this is a foolproof way of beating each level
1) start with 55555
2) for - drop that number to 3 / for + increase it to 7
3a) If you now have -- for a number decrease to 1 / if you have ++, increase to 9
3b) Otherwise (ie if the cumulative result is -+ or +-) the number is between the digits you entered
4) Repeat until you have all the numbers