13. Roman to Integer

题目地址：https://leetcode.com/problems/roman-to-integer/
思路：从左到右遍历，把当前字符对应数字累加到结果中，如果当前数比上一个数大，则需要减去上一个数的2倍。

class Solution {
public:
    int romanToInt(string s) {
        map<char, int> foo;
        foo['I'] = 1;
        foo['V'] = 5;
        foo['X'] = 10;
        foo['L'] = 50;
        foo['C'] = 100;
        foo['D'] = 500;
        foo['M'] = 1000;
        
        int result = 0;
        int len = s.length();
        for (int i=0; i<len; i++) {
            result += foo[(s[i])];
            if ((i-1 >=0) && (foo[s[i]] - foo[s[i-1]]) > 0) {
                result -= 2 * foo[s[i-1]];
            }
        }
        return result;
    }
};