Count and Say.cpp

/*
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
*/

class Solution
{
public:
  string countAndSay(int n)
  {
    if (n == 1)
      return "1";
    auto prev_ans = countAndSay(n - 1);

    string ans = "";
    int N = prev_ans.size(), count = 1;

    for (int i = 0; i < N; ++i)
    {
      if (i == N - 1)
      {
        ans += to_string(count);
        ans += prev_ans[i];
      }
      else if (prev_ans[i] == prev_ans[i + 1])
        count++;
      else
      {
        ans += to_string(count);
        ans += prev_ans[i];
        count = 1;
      }
    }

    return ans;
  }
};