Count and Say

Count and Say - LeetCodeLeetCode

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 substrings such that each substring contains exactly one unique digit. Then for each substring, say the number of digits, then say the digit. Finally, concatenate every said digit.

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

Given a positive integer n, return the nth 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

To generate the count-and-say sequence, we need to follow the recursive formula described. Here's a JavaScript implementation:

function countAndSay(n) {
    if (n === 1) return "1";

    let result = "1";

    for (let i = 2; i <= n; i++) {
        let current = "";
        let count = 1;

        for (let j = 1; j < result.length; j++) {
            if (result[j] === result[j - 1]) {
                count++;
            } else {
                current += count + result[j - 1];
                count = 1;
            }
        }

        current += count + result[result.length - 1];
        result = current;
    }

    return result;
}

// Test cases
console.log(countAndSay(1)); // Output: "1"
console.log(countAndSay(4)); // Output: "1211"

In this implementation, we start with the base case of n = 1, which returns "1". For each subsequent n, we iterate through the previous string and count consecutive digits, appending the count followed by the digit to the new string. Finally, we update the result for the next iteration.