Leetcode Algorithm

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

Hide Hint #1

The following are the terms from n=1 to n=10 of the count-and-say sequence:

1.     1
 2.     11
 3.     21
 4.     1211
 5.     111221 
 6.     312211
 7.     13112221
 8.     1113213211
 9.     31131211131221
10.     13211311123113112211

Hide Hint #2

To generate the nth term, just count and say the n-1th term.

Solution:

/**
 * @param {number} n
 * @return {string}
 */
var countAndSay = function(n){
    let finalString = '1';
    
    if (n == 1){
        return finalString;
    }
    let characterPointer = 0;
    let countPointer = 0;
    let stringInProgress = '';
    
    while (n > 1){
        while (countPointer < finalString.length){
            while(finalString.charAt(characterPointer)===finalString.charAt(countPointer)){
                countPointer ++;
                }
            
            stringInProgress += (countPointer - characterPointer).toString();
            stringInProgress += finalString.charAt(characterPointer);
            characterPointer = countPointer;
       }
    finalString = stringInProgress;
    stringInProgress = '';
    n --;
    characterPointer = 0;
    countPointer = 0;
    }
    return finalString;
};

Link

Reference