One of the test cases is looking for -c+5ab but the instructions say:

All monomials appears in order of increasing number of variables, e.g.:
"-abc+3a+2ac" -> "3a+2ac-abc", "xyz-xz" -> "-xz+xyz"

Confession time. This was basically my solution, however my logic says seen should start at 1 (as it will bounce down once and not come up again or forfeight and return -1), my test results were all coming back 2 more than the answer, so I changed seen to -1 and tested it to see if it would work with ALL the tests aaand it did. But I don't understand why... if someone could share some insight with me I would appreciate it very much