Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX

• I can be placed before V (5) and X (10) to make 4 and 9. 
• X can be placed before L (50) and C (100) to make 40 and 90. 
• C can be placed before D (500) and M (1000) to make 400 and 900.

Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

Input: "III"
Output: 3

Example 2:

Input: "IV"
Output: 4

Example 3:

Input: "IX"
Output: 9

Example 4:

Input: "LVIII"
Output: 58
Explanation: C = 100, L = 50, XXX = 30 and III = 3.

Example 5:

Input: "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

分析：

羅馬數字轉整數主要需要特別注意“IV、IX、XL、XC、CD、CM”這幾種情況，其餘情況直接加上羅馬字母對應的值即可。

class Solution {
public:
    int romanToInt(string s) {
        int res = 0;
        
        for(int i=0; i<s.size(); i++)
        {
           if(s[i] == 'V')
               res += 5;
           else if(s[i] == 'L')
               res += 50;
           else if(s[i] == 'D')
               res += 500;
           else if(s[i] == 'M')
               res += 1000;
            else if(s[i] == 'I')
           {
               if(s[i+1]=='V')
               {
                   res += 4;
                   i++;
               }
                   
               else if(s[i+1]=='X')
                {
                   res += 9;
                   i++;
               }
               else
                   res +=1;
           }
           else if(s[i] == 'X')
           {
               if(s[i+1]=='L')
                   {
                   res += 40;
                   i++;
               }
               else if(s[i+1]=='C')
                   {
                   res += 90;
                   i++;
               }
               else
                   res +=10;
           }
           else if(s[i] == 'C')
           {
               if(s[i+1]=='D')
                    {
                   res += 400;
                   i++;
               }
               else if(s[i+1]=='M')
               {
                   res += 900;
                   i++;
               }
               else
                   res +=100;
           }            
        }                        
        
        return res;
        
    }
};