Roman to Integer

Given a roman numeral, convert it to an integer.

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

The conversion algorithm from roman to integer is below:

if current char value is greater than the previous char value, result = result - previous + (current - previous). else, result = result + current;

public int charToInt(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 if (c=='M') {
            return 1000;
        }
        return 0;
    }

    public int romanToInt(String s) {
        if (s==null||s.isEmpty()) {
            return 0;
        }
        int result = charToInt(s.charAt(0));
        for (int i=1; i<s.length(); i++) {
            int current = charToInt(s.charAt(i));
            int previous = charToInt(s.charAt(i-1));
            if (current<=previous) {
                result += current;
            } else {
                //first remove already added previous value then do the subtraction
                result = (result-previous) + (current-previous);
            }
        }
        return result;
    }