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.

class Solution {public:    int romanToInt(string s) {        int length = s.length();        if(length <1) return 0;        map<char,int> m;        m['I'] = 1;        m['V'] = 5;        m['X'] = 10;        m['L'] = 50;        m['C'] = 100;        m['D'] = 500;        m['M'] = 1000;        int i = length-1;        int sum = m[s[i--]];        while(i>=0)            if(m[s[i+1]] > m[s[i]])                sum -= m[s[i--]];            else                sum += m[s[i--]];        return sum;    }};