map

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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 = 0;                
        for(int i = 0 ; i < s.size(); ++i)
            m[s[i]] < m[s[i+1]] ?   sum -= m[s[i]]:   sum += m[s[i]];
        return sum;
    }
};

数组(效率很高)

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class Solution {
public:
    int romanToInt(string s) {
        int m[90] = {};
        m['I'] = 1;
        m['V'] = 5;
        m['X'] = 10;
        m['L'] = 50;
        m['C'] = 100;
        m['D'] = 500;
        m['M'] = 1000;
        int sum = 0;                
        for(int i = 0 ; i < s.size(); ++i)
            m[s[i]] < m[s[i+1]] ?   sum -= m[s[i]]:   sum += m[s[i]];
        return sum;
    }
};