題目描述：

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.

C語言解法

int romanToInt(char* s) {
    /*
    *根據題目可知，遇到'I','X','C'羅馬字元需要判斷後面羅馬字元內容
    */
    int len = strlen(s);
    int sum = 0;
    int i = 0;
    char rom = s[0];
    
    for(i=0; i<len; i++)
    {
        switch(*(s+i)){
                
          case 'M':
                sum += 1000;
                break;
          case 'D':
                sum += 500;
                break;
          case 'C':
                (*(s+i+1) == 'D' || *(s+i+1) == 'M')?(sum-=100):(sum+=100);
                break;
          case 'L':
                sum += 50;
                 break;
          case 'X':
                (*(s+i+1) == 'L' || *(s+i+1) == 'C')?(sum-=10):(sum+=10);
                break;
          case 'V':
                sum+=5;
                break;
          case 'I':
                (*(s+i+1) == 'I' || *(s+i+1) == 0)?(sum++):(sum--);
                break;
        }
    }
    
    return sum;
    
}

執行結果：

Python3解法：

class Solution:
    def romanToInt(self, s):
        """
        :type s: str
        :rtype: int
        """
        
        
        Rom_dic = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}
        sum = Rom_dic[s[len(s)-1]]
        
        i = len(s) - 2
        while i >=0:
            if(Rom_dic[s[i]]<Rom_dic[s[i+1]]):
                sum -=Rom_dic[s[i]]
            else:
                sum += Rom_dic[s[i]]
                
            i = i-1
        
        return sum

執行結果：