RomanToInteger.java

package by.andd3dfx.numeric;

import java.util.Map;

/**
 * <pre>
 * <a href="https://leetcode.com/problems/roman-to-integer/description/">Task description</a>
 *
 * 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 ones 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.
 *
 * Example 1:
 * Input: s = "III"
 * Output: 3
 * Explanation: III = 3.
 *
 * Example 2:
 * Input: s = "LVIII"
 * Output: 58
 * Explanation: L = 50, V= 5, III = 3.
 *
 * Example 3:
 * Input: s = "MCMXCIV"
 * Output: 1994
 * Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
 *
 * Constraints:
 *     1 <= s.length <= 15
 *     s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
 *     It is guaranteed that s is a valid roman numeral in the range [1, 3999].
 * </pre>
 *
 * @see <a href="https://youtu.be/vlKjGPFyltU">Video solution</a>
 */
public class RomanToInteger {

    private static final Map<Character, Integer> vocabulary = Map.of(
            'I', 1,
            'V', 5,
            'X', 10,
            'L', 50,
            'C', 100,
            'D', 500,
            'M', 1000
    );

    public static int romanToInt(String s) {
        var sum = 0;
        var pos = 0;
        while (pos < s.length()) {
            var left = vocabulary.get(s.charAt(pos));
            if (pos == s.length() - 1) {
                sum += left;
                break;
            }

            var right = vocabulary.get(s.charAt(pos + 1));
            if (left >= right) {
                sum += left;
            } else {
                sum -= left;
            }
            pos++;
        }

        return sum;
    }
}