LeetCode 38. Count and Say

 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.

 

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 say(string s){
	string ans = "";
	int n = s.size();
	for(int i = 0; i < n; i++){
	char c = s[i];
	int cnt = 1;
	// iterate same chars
	while(i < n - 1 and s[i] == s[i+1]){
	cnt++;
	i++;
	}
	// int to string
	ans += to_string(cnt) + c;
	}
	return ans;
	}
	string countAndSay(int n) {
	string ans = "1";
	for(int i = 0; i < n - 1; i++){
	ans = say(ans);
	}
	return ans;
	}
	};

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