LeeCode-Roman to Integer

Given a roman numeral, convert it to an integer.

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

 1 class Solution {
 2 public:
 3     int romanToInt(string s) {
 4         int length = s.length();
 5         if(length <1) return 0;
 6         map<char,int> m;
 7         m[‘I‘] = 1;
 8         m[‘V‘] = 5;
 9         m[‘X‘] = 10;
10         m[‘L‘] = 50;
11         m[‘C‘] = 100;
12         m[‘D‘] = 500;
13         m[‘M‘] = 1000;
14         int i = length-1;
15         int sum = m[s[i--]];
16         while(i>=0)
17             if(m[s[i+1]] > m[s[i]])
18                 sum -= m[s[i--]];
19             else
20                 sum += m[s[i--]];
21         return sum;
22     }
23 };