Count and Say

ID:38

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.

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"

Idea

Recursion

If n=1, base case, return "1"

If not, recursively call the method; When get a intermediate string, like "21", get the count of each element in sequence and connect count+number together to form the result

Code

public String countAndSay(int n) {
        if(n == 1){
            return "1";
        }
        String curr = countAndSay(n-1);
        int currInd = 1;
        String base = curr.substring(0,1);
        int count = 1;
        String result = "";
        if(curr.length()==1){
            return "1"+base;
        }
        while(currInd<curr.length()){
            if(curr.substring(currInd, currInd+1).equals(base)){
                count++;
            }
            else{
                result += String.valueOf(count)+base;
                base = curr.substring(currInd, currInd+1);
                count=1;
            }
            currInd++;
        }
        result+=String.valueOf(count)+base;
        return result;
    }