6.js

/** ------------------------------------------------------------------------------
 *
 * 6. Zigzag Conversion
 * Topics: String
 * https://leetcode.com/problems/zigzag-conversion/description/
 *
 ------------------------------------------------------------------------------ */
/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function (s, numRows) {
  if (numRows === 1 || numRows >= s.length) return s

  const rows = Array.from({ length: numRows }, () => "")
  let curRow = 0
  let goingDown = false

  for (const char of s) {
    rows[curRow] += char
    if (curRow === 0 || curRow === numRows - 1) goingDown = !goingDown
    curRow += goingDown ? 1 : -1
  }

  return rows.join("")
}

console.log(convert("PAYPALISHIRING", 3))
console.log(convert("PAYPALISHIRING", 4))
console.log(convert("A", 1))