LeetCode Integer to Roman

class Solution {

private:

    const static char* pattern[];

    const static char* roman_digit[];

public:

    string intToRoman(int num) {

        if (num < 1 || num > 3999) return "";

        string res("");

        int i=0;

        while (num) {

            res = getSingleRomanDigit(num % 10, i++) + res;

            num /= 10;

        }

        return res;

    }



    string getSingleRomanDigit(int digit, int mag) {

        if (digit == 0) return "";

        if (digit > 9 || digit < 1 || mag > 3 || mag < 0) {

            return "FORMAT ERROR";

        }

        string res;

        int pi = 0;

        for (int i=0; (pi = pattern[digit][i]) != '\0'; i++) {

            pi = pi - '0';

            res.push_back(roman_digit[mag][pi]);

        }

        return res;

    }

};

const char* Solution::pattern[] =  {"x", "0", "00", "000", "01", "1", "10", "100", "1000", "02"};

const char* Solution::roman_digit[] = {"IVX", "XLC", "CDM", "M"};

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