# 121. A number sequence

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Here is a sequence:

`'1'`

, first number is 1-
`'11'`

, since previous number has One(1) 1 -
`'21'`

, since previous number has Two(2) 1s -
`'1211'`

, since previous number has One(1) 2 and One(1) 1 -
`'111221'`

, since previous number has One(1) 1, One(1) 2, Two(2) 1s -
`'312211'`

, since previous number has Three(3) 1s, Two(2) 2s, One(1) 1 - ....

As explained above, the sequence is generated by counting the digits of previous number.

Please create `getNthNum(n)`

to return the n-th number string in the sequence, n starts from 1.