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 roman2Int(char c) {
        switch (c) {
            case 'I':
                return 1;
            case 'V':
                return 5;
            case 'X':
                return 10;
            case 'L':
                return 50;
            case 'C':
                return 100;
            case 'D':
                return 500;
            case 'M':
                return 1000;
        }
        return 0;
    }

    public int romanToInt(String s) {
        if (s.length()==0) {
            return 0;
        }
        if (s.length()==1) {
            return roman2Int(s.charAt(0));
        }
        char c = s.charAt(0);
        int m = roman2Int(c);
        int n = romanToInt(s.substring(1));
        if (s.charAt(1)==c) {
            return m+n;
        }
        if (m>n) {
            return m + n;
        }
        if (m<n) {
            return n-m;
        }
        return 0;
    }
}