LeetCode. 13. Roman to Integer

import java.util.HashMap;
class Solution {
    public HashMap<Character, Integer> hm = new HashMap<>();
    public int romanToInt(String s) {
        int l = s.length(), sum = 0;
        String str;
        for(int i = l - 1; i >= 0; i--) {
            char c = s.charAt(i);
            if(hm.get(c) == null) {
                hm.put(c, i);
            }
        }

        for(int i = 0; i < l ; i++) {
            char c = s.charAt(i);
            if(c == 'I') sum += mul('V','X', i) * getNum(c);
            else if(c == 'X') sum += mul('L','C', i) * getNum(c);
            else if(c == 'C') sum += mul('D','M', i) * getNum(c);
            else sum += getNum(c);
        }
        return sum;
    }

    int mul(char x, char y, int i) {
        int max = Integer.MIN_VALUE;
        if(hm.containsKey(x)) max = hm.get(x);
        if(hm.containsKey(y)) max = Math.max(max, hm.get(y));
        return max < i ? 1 : -1;
    }

    boolean startWithOne(char c) { return (c == 'I' || c == 'X' || c == 'C') ? true : false; }

    int getNum(char c) {
        if(c == 'I') return 1;
        else if(c == 'V') return 5;
        else if(c == 'X') return 10;
        else if(c == 'L') return 50;
        else if(c == 'C') return 100;
        else if(c == 'D') return 500;
        else return 1000;
    }
}