Given an integer, convert it to a roman numeral.

intToRoman.cc

class Solution {
public:
    string intToRoman(int num) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.    
        /*Symbol 	Value
            I 	    1
            V 	    5
            X 	    10
            L 	    50
            C 	    100
            D 	    500
            M 	    1,000
        */
        static int tb[7] = {1,5,10,50,100,500,1000};
        static char sym[7] = {'I','V','X','L','C','D','M'};
        ostringstream oss;
        int curUnit = 6;
        while(curUnit>=0 && num) 
        {
            // special cases
            switch (curUnit)
            {
                case 5: // 500
                    if(num/900 == 1)
                    {
                        oss<<"CM";
                        num-=900;
                    }
                    break;
                case 4: // 100
                    if(num/400 == 1)
                    {
                        oss <<"CD";
                        num-=400;
                    }
                    break;
                case 3: // 50
                    if(num/90 == 1)
                    {
                        oss<<"XC";
                        num-=90;
                    }
                    break;
                case 2: // 10
                    if(num/40 == 1)
                    {
                        oss<<"XL";
                        num-=40;
                    }
                    break;
                case 1: // 5
                    if(num/9 == 1)
                    {
                        oss<<"IX";
                        num-=9;
                    }
                    break;
                case 0: // 1
                    if(num/4 == 1)
                    {
                        oss<<"IV";
                        num-=4;
                    }
                    break;
            }
            int count = num/tb[curUnit];
            for(int i = 0; i<count; ++i)
            {
                oss << sym[curUnit];
            }
            num = num - count*tb[curUnit];
            curUnit--;
        }
        
        return oss.str();
    }
};

romanToInt.cc

class Solution {
public:
    int romanToInt(string s) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
         /*Symbol Value
                I 1
                V 5
                X 10
                L 50
                C 100
                D 500
                M 1,000
        */
        /*static int tb[7] = {1,5,10,50,100,500,1000};
        static char sym[7] = {'I','V','X','L','C','D','M'};
        static int spec[6] = {4,9,40,90,400,900};
        static string specStr[6] = {"IV","IX","XL","XC","CD","CM"};*/
        int num = 0;
        for(size_t i = 0; i<s.length(); ++i)
        {
            char c = s[i];
            switch (c)
            {
                case 'M':
                    num += 1000;
                    break;
                case 'D':
                    num += 500;
                    break;
                case 'C':
                    // look ahead
                    if(i+1 < s.length())
                    {
                        if(s[i+1] == 'M')
                        {
                            num +=900;
                            i++;
                        }else if(s[i+1] == 'D')
                        {
                            num += 400;
                            i++;
                        }else
                        {
                            num += 100;
                        }
                    }else
                    {
                        num += 100;
                    }
                    break;
                case 'L':
                    num+=50;
                    break;
                case 'X':
                    // look ahead
                    if(i+1 < s.length())
                    {
                        if(s[i+1] == 'C')
                        {
                            num+=90;
                            i++;
                        }else if(s[i+1] == 'L')
                        {
                            num+=40;
                            i++;
                        }else
                        {
                            num+=10;
                        }
                    }else
                    {
                        num+=10;
                    }
                    break;
                case 'V':
                    num+=5;
                    break;
                case 'I':
                    // look ahead
                    if(i+1 < s.length())
                    {
                        if(s[i+1] == 'X')
                        {
                            num+=9;
                            i++;
                        }else if(s[i+1] == 'V')
                        {
                            num+=4;
                            i++;
                        }else
                        {
                            num++;
                        }
                    }else
                    {
                        num++;
                    }
                    break;
            }
        }
        
        return num;
    }
};