13. Roman to Integer

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, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II.

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. There are six instances where subtraction is used:

• 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.

Example 1:

Input: s = "III"
Output: 3

Example 2:

Input: s = "IV"
Output: 4

Example 3:

Input: s = "IX"
Output: 9

Example 4:

Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 5:

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

Constraints:

• 1 <= s.length <= 15
• s contains only the characters ('I', 'V', 'X', 'L', 'C', 'D', 'M').
• It is guaranteed that s is a valid roman numeral in the range [1, 3999].

class Solution {
public:
    int romanToInt(string s) {
        
        int weight[20] = {0};
        char symbol[7] = {'I', 'V', 'X', 'L', 'C', 'D', 'M'};
        
        int total = 0;
        
        for(int i=0; i<s.length(); i++){
            if(s[i] == 'I')
                weight[i] = 1;
            
            if(s[i] == 'V')
                weight[i] = 2;
            
            if(s[i] == 'X')
                weight[i] = 3;
            
            if(s[i] == 'L')
                weight[i] = 4;
            
            if(s[i] == 'C')
                weight[i] = 5;
            
            if(s[i] == 'D')
                weight[i] = 6;
            
            if(s[i] == 'M')
                weight[i] = 7;
        }
        
        for(int i=0; i<s.length(); i++){
            
            if(i+1 < s.length() && weight[i] < weight[i+1]){
                
                //special case
                if(s[i+1] == 'V'){
                    total += 4;
                }
                
                if(s[i+1] == 'X'){
                    total += 9;
                }
                
                if(s[i+1] == 'L'){
                    total += 40;
                }
                
                if(s[i+1] == 'C'){
                    total += 90;
                }
                
                if(s[i+1] == 'D'){
                    total += 400;
                }
                
                if(s[i+1] == 'M'){
                    total += 900;
                }
               i++;
            }
            else{
               
                if(s[i] == 'I')
                    total += 1;
                if(s[i] == 'V')
                     total += 5;
                if(s[i] == 'X')
                     total += 10;
                if(s[i] == 'L')
                     total += 50;
                if(s[i] == 'C')
                     total += 100;
                if(s[i] == 'D')
                     total += 500;
                if(s[i] == 'M')
                     total += 1000;
           } 
                        
        }                     
        return total;
    }
};

TIPS:

weight[20]:陣列內的20是我自己設的，輸入字串長度<20。

本來設weight[7]，得到下列錯誤訊息:

Line 35: Char 28: runtime error: index 7 out of bounds for type ‘int [7]’ (solution.cpp) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior prog_joined.cpp:44:28

special case: 權重正常左邊>右邊，若右邊>左邊則為special case，如"IV"。

Success

Runtime: 4 ms, faster than 89.69% of C++ online submissions for Roman to Integer.

Memory Usage: 6 MB, less than 71.81% of C++ online submissions for Roman to Integer.