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) {
         int val[ 256];
        val[ ' I ']= 1;
        val[ ' V ']= 5;
        val[ ' X ']= 10;
        val[ ' L ']= 50;
        val[ ' C ']= 100;
        val[ ' D ']= 500;
        val[ ' M ']= 1000;
         if(s.length()== 0)
             return  0;
         int num=val[s[ 0]];
         for( int i= 1;i<s.length();i++)
        {
             if(val[s[i]]<=val[s[i- 1]])
                num=num+val[s[i]];
             else
                num=num-val[s[i- 1]]-val[s[i- 1]]+val[s[i]];
        }
         return num;
    }
};