0038. Count and Say

https://leetcode.com/problems/count-and-say

Description

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 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

ac

class Solution {
    public String countAndSay(int n) {
        // edge cases
        if (n < 1) return "";

        //  generate
        String res = "1";
        for (int i = 1; i < n; i++) {
            res = counter(res);
        }

        return res;
    }
    private String counter(String s) {
        // input check
        if (s == null || s.length() < 1) return "";
        if (s.length() == 1) return "" + 1 + s;

        int cnt = 1;
        StringBuilder sb = new StringBuilder();
        for (int i = 1; i < s.length(); i++) {
            if (s.charAt(i) == s.charAt(i-1)) {
                cnt++;
            } else {
                sb.append(cnt);
                sb.append(s.charAt(i-1));
                cnt = 1;
            }
        }
            // final check
        sb.append(cnt);
        sb.append(s.charAt(s.length()-1));

        return sb.toString();
    }
}