13.Roman to Integer
===

### Description
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 ones 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
Explanation: III = 3.
#### example 2:
Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.
#### example 3:
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].

### 想法
相較於c，python在這種類似於hashmap的資料整理上較為整齊也方便，c的判別式我看起來比較直觀，寫起來比較順，而python要注意i<len(s)-1 這個判別式，這也可能是使用while 跟for loop 的差別

### 程式碼
#### c:

int charToInt(char ch){
    switch(ch){
        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;
        default : return 0;
    }
}

int romanToInt(char * s){
    if(s == NULL)
        return 0;
    int index = 0; // 索引
    int result = 0;// 最後輸出結果
    while(s[index] !='\0'){
        if(charToInt(s[index]) < charToInt(s[index+1])){
            result += charToInt(s[index+1]) - charToInt(s[index]);
            index++;            
        }else{
            result += charToInt(s[index]) ;
        }
        index++;
    }
    return result;
}

Runtime: 15 ms
Memory Usage: 6 MB
#### python:

class Solution:
    def romanToInt(self, s: str) -> int:
        a = {'I':1, 'V':5, 'X':10, 'L':50, 'C':100, 'D':500, 'M':1000}        
        ans=0        
        for i in range(len(s)):            
            if i<len(s)-1 and a[s[i]]<a[s[i+1]]:                
                ans-=a[s[i]]
            else:
                ans+=a[s[i]]
        return ans

Runtime: 57 ms
Memory Usage: 13.8 MB