I am doing the Roman to integer problem, but I don’t get why my code is working. I troubleshooted and for some reason doubles.containsKey(s.substring(i, i + 2)) is evaluating to false every time.

`import java.util.Map;

import java.util.Map;

class Solution {
    public int romanToInt(String s) {
        int res = 0;
        Map<String, Integer> map = new HashMap<>();
        map.put("I", 1);
        map.put("V", 5);
        map.put("X", 10);
        map.put("L", 50);
        map.put("C", 100);
        map.put("D", 500);
        map.put("M", 1000);

        Map<String, Integer> doubles = new HashMap<>();
        map.put("IV", 4);
        map.put("IX", 9);
        map.put("XL", 40);
        map.put("XC", 90);
        map.put("CD", 400);
        map.put("CM", 900);

        for (int i = 0; i < s.length(); i++) {
            String c = s.substring(i, i + 1);
            if (i != s.length() - 1 && doubles.containsKey(s.substring(i, i + 2))) {
                res += doubles.get(s.substring(i, i + 2));
                System.out.println(res);
                i++;
            } else {
                res += map.get(c);
            }
        }
        return res;
    }
}