RomanToInteger.java

package com.thealgorithms.conversions;

import java.util.*;

public class RomanToInteger {

    private static final Map<Character, Integer> ROMAN_TO_INT = new HashMap<>() {
        {
            put('I', 1);
            put('V', 5);
            put('X', 10);
            put('L', 50);
            put('C', 100);
            put('D', 500);
            put('M', 1000);
        }
    };

    // Roman Number = Roman Numerals

    /**
     * This function convert Roman number into Integer
     *
     * @param A Roman number string
     * @return integer
     */
    public static int romanToInt(String A) {
        A = A.toUpperCase();
        char prev = ' ';

        int sum = 0;

        int newPrev = 0;
        for (int i = A.length() - 1; i >= 0; i--) {
            char c = A.charAt(i);

            if (prev != ' ') {
                // checking current Number greater then previous or not
                newPrev = ROMAN_TO_INT.get(prev) > newPrev ? ROMAN_TO_INT.get(prev) : newPrev;
            }

            int currentNum = ROMAN_TO_INT.get(c);

            // if current number greater then prev max previous then add
            if (currentNum >= newPrev) {
                sum += currentNum;
            } else {
                // subtract upcoming number until upcoming number not greater then prev max
                sum -= currentNum;
            }

            prev = c;
        }

        return sum;
    }

    public static void main(String[] args) {
        int sum = romanToInt("MDCCCIV");
        System.out.println(sum);
    }
}