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/**
 * 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) {
        HashMap<Character, Integer> dic = new HashMap<Character, Integer>();
        dic.put('I', 1);
        dic.put('V', 5);
        dic.put('X', 10);
        dic.put('L', 50);
        dic.put('C', 100);
        dic.put('D', 500);
        dic.put('M', 1000);
        //start from the rightmost one. 
        int res = dic.get(s.charAt(s.length() - 1));
        for(int i = s.length() - 2; i >= 0; i--){
            if(dic.get(s.charAt(i + 1)) <= dic.get(s.charAt(i)))
                res += dic.get(s.charAt(i));
            else
                res -= dic.get(s.charAt(i));
        }
        return res;
    }
}