題目解說

原題目

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

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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:

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Input: s = "III"
Output: 3
Explanation: III = 3.

Example 2:

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

Example 3:

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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].

翻譯

這題主要是在說羅馬數字的計算方式，並告知('I', 'V', 'X', 'L', 'C', 'D', 'M') 分別等於數字多少，且 4 不是 IIII 而是 VI ，以此類推應用到 9,40,90,400,900。

解題策略

解題邏輯是讓每個字母等於一個數字，再用 object ={key:value} 的方式加總就好，首先要處理的就是 4,9,40,90,400,900 ，我宣告的 object 把 'IV':4,'IX':9,'XL':40,'XC':90,'CD':400,'CM':900 用一個字母取代掉 'f':4,'n':9,'l':40,'c':90,'d':400,'m':900，接著是做字串s的文字取代 s.replace('IV','f').replace('IX','n').replace('XL','l').replace('XC','c').replace('CD','d').replace('CM','m')，最後利用迴圈加上 charAt()就可以逐一取值並加總。

JS程式碼

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var romanToInt = function(s) {
    //'IV':4,'IX':9,'XL':40,'XC':90,'CD':400,'CM':900
    const obj  ={'I':1,'V':5,'X':10,'L':50,'C':100,'D':500,'M':1000,'f':4,'n':9,'l':40,'c':90,'d':400,'m':900};
    let new_s = s.replace('IV','f').replace('IX','n').replace('XL','l').replace('XC','c').replace('CD','d').replace('CM','m')
    let sum = 0
    for(let i = 1;i<=new_s.length;i++){
       sum = sum + obj[new_s.charAt(i-1)]
    }
    return sum;
};