Count and Say

Problem

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

countAndSay(1) = "1"
countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the n^th term of the count-and-say sequence.

Example 1:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

Constraints:

1 <= n <= 30

Solution

/**
 * @param {number} n
 * @return {string}
 */
var countAndSay = function(n) {
    let s = "1";
    for (let i = 1; i < n; ++i) {
        let digit = s[0];
        let count = 1;
        let output = "";
        for (let j = 1; j < s.length; ++j) {
            if (s[j] !== digit) {
                output += count.toString() + digit;
                count = 1;
                digit = s[j];
            } else {
                count++;
            }
        }
        output += count.toString() + digit;
        s = output;
    }
    return s;
};

Keep on calculating the next string in the sequence until reaching the nth iteration.

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