solutions/0038-count-and-say.js

/**
 * 38. Count and Say
 * https://leetcode.com/problems/count-and-say/
 * Difficulty: Medium
 *
 * 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.
 */

/**
 * @param {number} n
 * @return {string}
 */
var countAndSay = function(n) {
  let result = '1';

  for (let i = 1; i < n; i++) {
    result = result.replace(/((\d)\2*)/g, '$1—')
                   .split('—')
                   .map(s => s ? `${s.length}${s[0]}` : '')
                   .join('');
  }

  return result;
};