LeetCode 13. Roman to Integer

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class Solution {
unordered_map<char, int> m = {
    {'I', 1},
    {'V', 5},
    {'X', 10},
    {'L', 50},
    {'C', 100},
    {'D', 500},
    {'M', 1000},
};
public:
    int romanToInt(string s) {
        const int n = s.size();
        int ans = 0;
        for (int i = 0; i < n; ++i) {
            int v = m[s[i]];
            if (i < n - 1 && v < m[s[i + 1]]) ans -= v;
            else ans += v;
        }
        return ans;
    }
};