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.

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把罗马数字转成阿拉伯整数

C++（85ms）：

class Solution {
public:
    int romanToInt(string s) {
        unordered_map<char,int> m ={{'I',1},
                                    {'V',5},
                                    {'X',10},
                                    {'L',50},
                                    {'C',100},
                                    {'D',500},
                                    {'M',1000},};

        int sum = m[s.back()] ;
        for (int i = s.size()-2; i >= 0;i-- ){
            if (m[s[i]] < m[s[i+1]])
                sum -= m[s[i]] ;
            else
                sum += m[s[i]] ;
        }
        return sum ;
    }
};