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. Following is an example to understand the problem better:

The Count And Say Problem

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) {
        
        String result = "1";
        if(n == 1){
            return result;
        }
        for(int i = 2 ; i <= n ; i++){
            result = countAndSay(result);
        }
        return result;
    }
    
    private String countAndSay(String s){
        char prev = s.charAt(0);
        int count = 1;
        int n = s.length();
        StringBuilder result = new StringBuilder();
        for(int i = 1 ; i < n ; i++){
            char cur = s.charAt(i);
            if(prev == cur){
                count++;
            }else{
                result.append(count);
                result.append(prev);
                count = 1;
                prev = cur;
            }
        }
        result.append(count);
        result.append(prev);
        return result.toString();
    }
}

Categories: String

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