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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0038_count_and_say;

// #Medium #Top_Interview_Questions #String #2023_08_09_Time_2_ms_(97.68%)_Space_39.9_MB_(90.19%)

/**
 * 38 - Count and Say\.
 *
 * Medium
 *
 * 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`
**/
public class Solution {
    public String countAndSay(int n) {
        if (n == 1) {
            return "1";
        }
        StringBuilder res = new StringBuilder();
        String prev = countAndSay(n - 1);
        int count = 1;
        for (int i = 1; i < prev.length(); i++) {
            if (prev.charAt(i) == prev.charAt(i - 1)) {
                count++;
            } else {
                res.append(count).append(prev.charAt(i - 1));
                count = 1;
            }
        }
        res.append(count).append(prev.charAt(prev.length() - 1));
        return res.toString();
    }
}