RomanToInteger.java

/**
 * 13. 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, 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.
 */

class RomanToInteger {
    public int romanToInt(String theRoman) {
        Map<Character, Integer> symbolValues = new HashMap<>();
        symbolValues.put('I', 1);
        symbolValues.put('V', 5);
        symbolValues.put('X', 10);
        symbolValues.put('L', 50);
        symbolValues.put('C', 100);
        symbolValues.put('D', 500);
        symbolValues.put('M', 1000);

        int result = 0;
        int prevValue = 0;

        for (int i = theRoman.length() - 1; i >= 0; i--) {
            int currValue = symbolValues.get(theRoman.charAt(i));

            if (currValue < prevValue)
                result -= currValue;
            else {
                result += currValue;
                prevValue = currValue;
            }
        }
        return result;
    }
}