1 = one 1 = next number is 11 11 = two 1's = next number is 21 21 = one 2, one 1 = next number is 1211 1211 = one 1 one 2 two 1 = next number is 111221 111221 = three 1 two 2 one 1 = next number is 312211
AS each number tell about previous number, so next number will be 312211