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.

class Solution {
public:
    int romanToInt(string s) {
        /*
        I   V   X   L   C   D   M
        1   5   10  50  100 500 1000
        */
        int res = 0;
        int i = 0, n = s.size();
        while ( i <= n-1 ) {
            char c1 = s[i], c2 = ' ';
            if ( i <= n-2 ) c2 = s[i+1];
            if ( c1 == 'M' ) res += 1000;
            if ( c1 == 'D' ) res += 500;
            if ( c1 == 'C' ) {
                if ( c2 == 'D') {res += 400; i += 1;}
                if ( c2 == 'M') {res += 900; i += 1;}
                if ( c2 != 'D' and c2 != 'M' ) res += 100;
            }
            if ( c1 == 'L' ) res += 50;
            if ( c1 == 'X' ) {
                if ( c2 == 'L') {res += 40; i += 1;}
                if ( c2 == 'C') {res += 90; i += 1;}
                if ( c2 != 'L' and c2 != 'C' ) res += 10;
            }
            if ( c1 == 'V' ) res += 5;
            if ( c1 == 'I' ) {
                if ( c2 == 'V') {res += 4; i += 1;}
                if ( c2 == 'X') {res += 9; i += 1;}
                if ( c2 != 'V' and c2 != 'X' ) res += 1;
            }
            i += 1;
        }
        return res;
    }
};

Note: it is illegal to define an empty character:

char c = '';