# 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"`:

![](https://assets.leetcode.com/uploads/2020/10/23/countandsay.jpg) 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

```java
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();
    }
}
```


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