Integer to Roman & Roman to Integer

class Solution {
public:
    const string romans[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 intToRoman(int num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function    
        int p=0;
        string res="";
        while (num>0) {
            int n=num%10;
            num/=10;
            res=romans[p][n]+res;
            p++;
        }
        return res;
    }
};

1/17/14:

greedy:

class Solution {
public:
    string intToRoman(int num) {
        string roman[] = {"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};    
        int nums[] = {1000,900,500,400,100,90,50,40,10,9,5,4,1};
        string res="";
        int cur=num;
        int i=0;
        while (cur>0) {
            for (; i<13; i++) {
                if (cur>=nums[i]) {
                    break;
                }
            }
            res.append(roman[i]);
            cur -=nums[i];
        }
        return res;
    }
};

Roman to Integer

class Solution {
public:
    const string romans[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", "", "", "", "", "", ""}  
    };
    int singleValue(char c) {
        switch (c) {
            case 'I': return 1;
            case 'V': return 5;
            case 'X': return 10;
            case 'L': return 50;
            case 'C': return 100;
            case 'D': return 500;
            case 'M': return 1000;
        }
    }
    int romanToInt(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int res=0;
        int i=0;
        int pre=singleValue(s[0]);
        for (; i<s.size(); i++) {
            int now=singleValue(s[i]);
            if (pre<now) {
                res=res-2*pre+now;
            }else {
                res+=now;
            }
            pre=now;
            
        }
        return res;
    }
};

1/17/14: traverse backwards, if current one is smaller than the previous one, subtract the current number form the result, otherwise, add it to the result.

class Solution {
public:
    int romanToInt(string s) {
        map<char, int> m;
        m['M'] = 1000;
        m['D'] = 500;
        m['C'] = 100;
        m['L'] = 50;
        m['X'] = 10;
        m['V'] = 5;
        m['I'] = 1;
        
        int res=m[s[s.size()-1]];
        for (int i=s.size()-2; i>=0; i--) {
            if (m[s[i]]>=m[s[i+1]]) {
                res +=m[s[i]];
            }else {
                res -=m[s[i]];
            }
        }
        return res;
    }
};