面试题36：罗马数和阿拉伯数的相互转换

class Solution {
public:
    int romanToInt(string s) {
        unordered_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 res = 0;
        int pre = 0;
        int len = s.length();
        for(int i=len-1;i>=0;i--){
            int a = m[s[i]];
            if(a >= pre){
                pre = a;
                res += a;
            }else{
                pre = a;
                res -= a;
            }
        }
        return res;
    }
};
class Solution {
public:
    string intToRoman(int num) {
        char* c[4][10]={
            {"","I","II","III","IV","V","VI","VII","VIII","IX"},
            {"","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"},
            {"","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"},
            {"","M","MM","MMM"}
        };
        string roman;
        roman.append(c[3][num / 1000 % 10]);
        roman.append(c[2][num / 100 % 10]);
        roman.append(c[1][num / 10 % 10]);
        roman.append(c[0][num % 10]);
        
        return roman;
    }
};