12. 整数转罗马数字

/**
 * @param {number} num
 * @return {string}
 */
var intToRoman = function (num) {
  if (num < 1 || num > 3999) {
    return
  }
  const map = {
    1: 'I',
    5: 'V',
    10: 'X',
    50: 'L',
    100: 'C',
    500: 'D',
    1000: 'M',
  }
  let result = ''
  let digtal = 10
  while (num > 0) {
    const n = num % 10
    if (n <= 3) {
      result = map[digtal / 10].repeat(n) + result
    } else if (n === 4) {
      result = map[digtal / 10] + map[digtal / 2] + result
    } else if (n === 5) {
      result = map[digtal / 2] + result
    } else if (n <= 8) {
      result = map[digtal / 2] + map[digtal / 10].repeat(n % 5) + result
    }
    if (n === 9) {
      result = map[digtal / 10] + map[digtal] + result
    }
    num = Math.floor(num / 10)
    digtal *= 10
  }
  return result
}