Find the next number in the following series

Question

1 11 21 1211 111221 312211

Answer 1

1 -> ‘one 1’ -> 11 -> ‘two 1(s)’ -> 21 -> ‘one 2, one 1’ -> 1211 -> ‘one 1, one 2, two 1(s)’ -> 111221 and so on.

This is not a programming question though, and doesn’t belong here.