roman-to-integer.js

/**
 * Roman to Integer
 *
 * 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, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which
 * is simply X + II. The number twenty seven 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. Input is guaranteed to be within the range from 1 to 3999.
 *
 * Example 1:
 *
 * Input: "III"
 * Output: 3
 *
 * Example 2:
 *
 * Input: "IV"
 * Output: 4
 *
 * Example 3:
 *
 * Input: "IX"
 * Output: 9
 *
 * Example 4:
 *
 * Input: "LVIII"
 * Output: 58
 * Explanation: C = 100, L = 50, XXX = 30 and III = 3.
 *
 * Example 5:
 *
 * Input: "MCMXCIV"
 * Output: 1994
 * Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
 */

/**
 * @param {string} s
 * @return {number}
 */
const romanToInt = s => {
  if (!s || s.length === 0) {
    return 0;
  }

  const map = new Map([['I', 1], ['V', 5], ['X', 10], ['L', 50], ['C', 100], ['D', 500], ['M', 1000]]);

  let i = s.length - 1;
  let result = map.get(s[i]);

  while (i > 0) {
    const curr = map.get(s[i]);
    const prev = map.get(s[i - 1]);

    if (prev >= curr) {
      result += prev;
    } else {
      result -= prev;
    }

    i--;
  }

  return result;
};

export { romanToInt };