A limitation of 10000 is weak. My "stupid" O(n^2) PASCAL solution gets AC in 0.078 sec.
It works very fast even on its worth test:
aaaa....aaaab (9999 letters 'a')
where it makes 9999 iterations with = and Delete() with strings of length 10000.

But actually I don't know how to change this. If just increase  limitation, even correct solutions gets WA...

Maybe it is actual to make version 2 of problem, with limitation in 100000 or 250000.