Count and Say

Count and Say

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

Solutions

🧠 Cpp

class Solution
{
    string rle(string s)
    {
        size_t counter = 0;
        char current = s.front();
        string res;

        for(char ch : s)
        {
            if(ch == current)
                counter++;
            else
            {
                res += to_string(counter) + current;
                current = ch;
                counter = 1;
            }
        }

        //add data from last iteration at the end
        return res + to_string(counter) + current;

    }

public:
    string countAndSay(int n)
    {
        string res = "1";

        //run-length encoding
        while(--n)
            res = rle(res);

        return res;
    }
};