link on problem: 898C - Phone Numbers
I sent this submission 298630099, it didn't pass the 5th test and gave an execution error. This solution 298631384 has passed all the tests, I don't see any differences between them except for the structure of the input data storage, who can tell?
help^-^
In your ispref function, it could be that
x.size() > y.size()
theny[i]
will cause a RTE.