RomanToInteger.java

package Lc13_RomanToInteger;

import java.util.HashMap;
import java.util.Map;

/**
 * 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.
 *
 * 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].
 */
public class RomanToInteger {

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

    public int romanToInt(String s) {
        int ans = 0;
        for (int i = 0; i < s.length(); i++) {
            if (i == s.length() - 1) {
                ans += symbolValues.get(s.charAt(i));
                break;
            }

            Integer prev = symbolValues.get(s.charAt(i));
            Integer next = symbolValues.get(s.charAt(i + 1));
            if (prev < next) {
                ans -= prev;
            } else {
                ans += prev;
            }
        }
        return ans;
    }
}