[13] 罗马数字转整数

// @lc code=start
/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function (s) {
  const getValue = (a) => {
    switch (a) {
      case 'I':
        return 1;
      case 'V':
        return 5;
      case 'X':
        return 10;
      case 'L':
        return 50;
      case 'C':
        return 100;
      case 'D':
        return 500;
      case 'M':
        return 1000;
      default:
        return 0;
    }
  }
  const sl = s.length;
  let max = 1;
  let ans = 0;
  for (let i = sl - 1; i >= 0; i--) {
    const val = getValue(s.charAt(i));
    if (max > val) {
      ans -= val;
    } else {
      max = val;
      ans += val;
    }
  }
  return ans;
};