0482. License Key Formatting.js

// You are given a license key represented as a string S which consists only alphanumeric character and dashes. The string is separated into N+1 groups by N dashes.
//
// Given a number K, we would want to reformat the strings such that each group contains exactly K characters, except for the first group which could be shorter than K, but still must contain at least one character. Furthermore, there must be a dash inserted between two groups and all lowercase letters should be converted to uppercase.
//
// Given a non-empty string S and a number K, format the string according to the rules described above.
//
// Example 1:
// Input: S = "5F3Z-2e-9-w", K = 4
//
// Output: "5F3Z-2E9W"
//
// Explanation: The string S has been split into two parts, each part has 4 characters.
// Note that the two extra dashes are not needed and can be removed.
//
// Example 2:
// Input: S = "2-5g-3-J", K = 2
//
// Output: "2-5G-3J"
//
// Explanation: The string S has been split into three parts, each part has 2 characters except the first part as it could be shorter as mentioned above.
//
// Note:
// The length of string S will not exceed 12,000, and K is a positive integer.
// String S consists only of alphanumerical characters (a-z and/or A-Z and/or 0-9) and dashes(-).
// String S is non-empty.

/**
 * @param {string} S
 * @param {number} K
 * @return {string}
 */
const licenseKeyFormatting = (S, K) => {
  const s = S.replace(/-/g, '').toUpperCase();
  let len = s.length;
  const res = [];
  while (len > 0) {
    res.push(s.substring(len - K, len)); // do not use slice here, since substring works when start index < 0, it will return the string starting from 0
    len -= K;
  }
  return res.reverse().join('-');
};