0013-roman-to-integer.js

/**
 * 13. Roman to Integer
 * https://leetcode.com/problems/roman-to-integer/
 * Difficulty: Easy
 *
 * Roman numerals are represented by seven different symbols: `I`, `V`, `X`,
 * `L`, `C`, `D` and `M`.
 *
 * Symbol       Value
 * I            1
 * V            5
 * X            10
 * L            50
 * C            100
 * D            500
 * M            1000
 *
 * For example, `2` is written as `II` in Roman numeral, just two one's
 * added together. `12` is written as `XII`, which is simply `X + II`.
 * The number `27` is written as `XXVII`, which is `XX + V + II`.
 *
 * Roman numerals are usually written largest to smallest from left to right.
 * However, the numeral for four is not `IIII`. Instead, the number four is
 * written as `IV`. Because the one is before the five we subtract it making
 * four. The same principle applies to the number nine, which is written as
 * `IX`. There are six instances where subtraction is used:
 *
 * - `I` can be placed before `V (5)` and `X (10)` to make 4 and 9.
 * - `X` can be placed before `L (50)` and `C (100)` to make 40 and 90.
 * - `C` can be placed before `D (500)` and `M (1000)` to make 400 and 900.
 *
 * Given a roman numeral, convert it to an integer.
 */

/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function(s) {
  const map = { M: 1000, D: 500, C: 100, L: 50, X: 10, V: 5, I: 1 };

  return s.split('').reduce((result, v, i) => {
    result += map[v] >= map[s[i + 1]] || s.length === i + 1
      ? map[v]
      : -1 * map[v];
    return result;
  }, 0);
};