A number sequence

Question

Here is a sequence:

1. '1', first number is 1
2. '11', since previous number has One(1) 1
3. '21', since previous number has Two(2) 1s
4. '1211', since previous number has One(1) 2 and One(1) 1
5. '111221', since previous number has One(1) 1, One(1) 2, Two(2) 1s
6. '312211', since previous number has Three(3) 1s, Two(2) 2s, One(1) 1
7. ....

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.

Code

/**
 * @param {number} n - integer
 * @returns {string}
 */
function getNthNum(n) {
  // your code here
}

Source

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