ZigZag Conversion

Description

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:
P     I    N
A   L S  I G
Y A   H R
P     I

Solution(javascript)

/*
 * @lc app=leetcode id=6 lang=javascript
 *
 * [6] ZigZag Conversion
 */

// @lc code=start
/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
const convert = function (s, numRows) {
  if (numRows === 1) {
    return s
  }
  const arr = new Array(numRows).fill("")
  let i = 0
  let down = true
  for (const c of s) {
    arr[i] += c
    if (down) {
      i += 1
      if (i === numRows - 1) {
        down = false
      }
    } else {
      i -= 1
      if (i === 0) {
        down = true
      }
    }
  }
  return arr.join('')
}
// @lc code=end