12. Integer to Roman

首先建立一个数组，保持1 到3999的罗马数，以一位单位（0,1,2,3,4,5,6,7,8,9），
10为单位(0,10,20,30,40,50,60,70,80,90)
100为单位(0,100,200,300,400,500,600,700,800,900)
1000为单位(0,1000,2000,3000)

num/1000为index从(0,1000,2000,3000)找
num/100%10 从(0,100,200,300,400,500,600,700,800,900)
num/10%10 从(0,10,20,30,40,50,60,70,80,90)
num%10 从（0,1,2,3,4,5,6,7,8,9）找

char* intToRoman(int num) {
    char *result = (char*)malloc(sizeof(char)*100);
    char one[][5]   = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
    char two[][5]   = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
    char three[][5] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
    char four[][5]  = {"", "M", "MM", "MMM"};
    
    result[0] = '\0';
    strcat(result, four[num/1000]);
    //num %= 1000;
    strcat(result, three[num/100%10]);
    //num %= 100;
    strcat(result, two[num/10%10]);
    //num %= 10;
    strcat(result, one[num%10]);
    
    return result;
}