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) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        HashMap<Character, 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);

        int result = map.get(s.charAt(s.length() - 1));
        for (int i = s.length() -2; i >= 0; i--) {
            if (map.get(s.charAt(i)) >= map.get(s.charAt(i + 1))) {
                result += map.get(s.charAt(i));
            } else {
                result -= map.get(s.charAt(i));
            }
        }
        return result;
    }
}
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