[LeetCode] 13. Roman to Integer

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() < 1) {
            throw new IllegalArgumentException("empty string");
        }
        int r = 0;
        int lastAdd = Integer.MAX_VALUE;
        for (int i = 0; i < s.length(); i++) {
            int t = charToInt(s.charAt(i));
            if (lastAdd < t) {
                r -= 2 * lastAdd;
            }
            r += t;
            lastAdd = t;
        }
        return r;
    }
    
    private int charToInt(char c) {
        int r = 0;
        switch (c) {
            case 'I':
                r = 1;
                break;
            case 'X':
                r = 10;
                break;
            case 'C':
                r = 100;
                break;
            case 'M':
                r = 1000;
                break;
            case 'V':
                r = 5;
                break;
            case 'L':
                r = 50;
                break;
            case 'D':
                r = 500;
                break;
            default:
                throw new IllegalArgumentException("out of range");
        }
        return r;
    }
}