class Solution {
    public int romanToInt(String s) {
        Map<Character, Integer> dict = new HashMap();
        dict.put('I',1);
        dict.put('V',5);
        dict.put('X',10);
        dict.put('L',50);
        dict.put('C',100);
        dict.put('D',500);
        dict.put('M',1000);
        int res = 0;
        int cur = s.length() -1;
        // pre指针总是向左走, cur与pre之间持[0,1]的距离.
        for(int pre = s.length() - 1; pre >= 0; pre--){
            if(dict.get(s.charAt(pre)) < dict.get(s.charAt(cur))){
                res -= dict.get(s.charAt(pre));
                cur = pre - 1;
            }else{
                res += dict.get(s.charAt(pre));
                cur = pre;
            }
        }
        return res;
    }
}