LeetCode-in-Java

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":

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

Solution

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();
    }
}

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