I may have not completely understood the instructions, but why would solve("czaccb") give "czab" instead of "zacb"?
I naively thought that you should use a rolling algorithm to remove each duplicate beginning on the left side :

czaccb -> _za_cb -> zacb

edit : this algorithm works with the second example

geeegfffgdadd -> _eee_fffgdadd -> eeefffgdadd -> __efffgdadd -> efffgdadd -> e__fgdadd -> efgdadd -> efg_a_d -> efgad

What am I getting wrong?