numSequence.js

/*
https://bigfrontend.dev/problem/A-number-sequence

121. A number sequence

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.
*/


/**
 * @param {number} n - integer
 * @returns {string}
 */
function getNthNum(n) {
    let nthNum = "1";

    while (--n) {
        let res = "";

        let char = nthNum[0], charCount = 1;
        for (let i = 1; i <= nthNum.length; i++) {
            if (nthNum[i] === char) {
                charCount++;
            } else {
                res += `${charCount}${char}`;
                char = nthNum[i];
                charCount = 1;
            }
        }

        nthNum = res;
    }

    return nthNum;
}

// console.log(getNthNum(1))
console.log(getNthNum(2))