Z字型输出
var convert = (s, rows) => {
if (rows === 1) return s
const row = new Array(rows).fill('')
const cutNum = 2 * rows - 2
for (let i = 0; i < s.length; i++) {
const x = i % cutNum
row[Math.min(x, cutNum - x)] += s[i]
}
return row.join('')
}
