38. 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.

class Solution {
public:
    string countAndSay(int n) {
        if(n == 1) return "1";
        string res = countAndSay(n - 1);
        stringstream ss;
        int cnt = 1;
        for(int i = 0; i < res.length() - 1; i++) {
            if(res[i] == res[i + 1]) cnt++;
            else {
                ss << to_string(cnt) << res[i];
                cnt = 1;
            }
        }
        return ss.str() + to_string(cnt) + res.back();
    }
};