Count and Say

Problem

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 substrings such that each substring contains exactly one unique digit. Then for each substring, say the number of digits, then say the digit. Finally, concatenate every said digit.

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 & Analysis

Code

Java

public class Solution {
    public String countAndSay(int n) {
        if (n == 1) return "1";

        int j = 0;

        String str = "11";

        for (int i = 0; i < n - 2; i++) {
            int count = 1;
            String s = "";

            for (j = 0; j < str.length() - 1; j++) {
                if (str.charAt(j) == str.charAt(j + 1))
                    count++;
                else {
                    s += Integer.toString(count); // count
                    s += str.charAt(j);           // say
                    count = 1;
                }
            }
            s += Integer.toString(count);
            s += str.charAt(j);
            str = s;
        }
        return str;
    }
}

python

class Solution:
    def countAndSay(self, n: int) -> str:

        answer='1'
        
        if n==1 : return answer
        
        for i in range(2, n+1):
            #read
            
            idx=0
            count=0
            temp=answer
            answer=''
            
            print(len(temp))
            for t in range (len(temp)):
            
                if t==0: idx=0
                  
                if temp[idx]==temp[t]:
                    count+=1
                
                else :
                    answer=answer+str(count)+temp[idx]
                    idx=t
                    count=1
                    
                print(temp[idx],temp[t])
                    
            answer=answer+str(count)+temp[idx]
                    
               
        
        return answer