leetcode-6.js

var convert = function(s, numRows) {
  if (numRows === 1) return s

  const mat = buildMatrix(s, numRows)
  return buildResult(mat)
};

function buildMatrix(s, numRows) {
  const mat = initMatrix(numRows)
  let r = 0, c = 0, down = true
  for (let i=0; i < s.length; i++) {
    mat[r][c] = s[i]
    
    if (down && r === mat.length - 1) {
      down = false
    } else if (!down && r === 0) {
      down = true
    }

    if (down) {
      r++
    } else {
      r--
      c++
    }
  }
  return mat
}

function initMatrix(numRows) {
  const mat = []
  for (let i=0; i < numRows; i++) {
    mat[i] = []
  }
  return mat
}

function buildResult(mat) {
  let result = ""
  for (let i=0; i < mat.length; i++) {
    for (let j=0; j < mat[i].length; j++) {
      if (mat[i][j]) result += mat[i][j]
    }
  }

  return result
}