Solve dis riddle ;)

[rpkash]

Here is a series, can anyone find the next string of the series below?

1
11
21
1211
111221
?

Regards,
CEAN .

[Elisa]

The First number is :
1 : as there is: one 1
the second number will be : 11 : as there is ( two 1)
the the third number is : 21 : as there is (One 2) (One 1)
the the fourth number is : 1211 : as there is ( one 1) and (one 2) and (two 1)
the fifth number is : 111221 as there is : (One 1) (one 2) (two 1)
the series continues like this:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221

[rpkash]

Elisa, , that was really a prompt reply . It took me 2 hours to come out of the result and that too when given a hint .
, u rock .
Regards,
.