LeetCode Roman to Integer

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

public class Solution {
    public int romanToInt(String s) {
        int res=0;
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        map.put("M", 1000);map.put("CM", 900);map.put("D", 500);map.put("CD", 400);
        map.put("C", 100);map.put("XC", 90);
        map.put("L", 50);map.put("XL", 40);map.put("X", 10);map.put("IX", 9);map.put("V", 5);
        map.put("IV", 4);map.put("I",1);
        if (s.equals("")) return 0;
        if (s.length()==1) return map.get(s);
        String ss;
        for (int i = 0; i < s.length(); i++) {
            if (i<s.length() - 1) {
                ss = s.substring(i, i + 2);
            }else {
                ss = s.substring(i, i + 1);
            }

            if (map.containsKey(ss)) {
                res = res + map.get(ss);
                ++i;
            } else {
                ss = s.substring(i, i + 1);
                res = res + map.get(ss);
            }
        }
        return res;
    }
}