The statement says "The only line of the output should contain the Average LCP of T with 3 digits after decimal point. "
Is this with rounding or without? I'm using printf("%0.3lf") and keep on getting WA 35

Can somebody tell what's the reason of WA 35?
I think it's not rounding.

Do you use long long to count total length of common prefixes? Try a test like AAA...AAA (250000 times)

I've tried both long long and double. There's no problem with such test.