That's stylish!

I like that imperative-style coding)

Test Passed
Test Passed
'5ab-c' should equal '-c+5ab'
Test monomial length ordering
'-abc+3a+2ac' should equal '3a+2ac-abc'
'xyz-xz' should equal '-xz+xyz'
Test lexicographic ordering
'a+ac-ab' should equal 'a-ab+ac'
'xyz+byz' should equal 'byz+xyz'
Test no leading +
'-y+x' should equal 'x-y'
'y-x' should equal '-x+y'

As for me I solved it.

UPD

But Anyways I've come up with it. Just added sorting in some places)))