Leetcode - Easy - 13. Roman to Integer - Javascript

Leetcode - Easy - 13. Roman to Integer - Javascript

   清晨6:14       Rain Lin

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13. Roman to Integer

Easy

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
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].

解題方向：

1. 這邊主要是要搞懂 Roman 規則, 再擬定羅馬與數字對照表

    a. 1 ~ 3 是用 I 表示

    b. 4 ~ 8 是與 V 相關

    c. 9 ~ 10 是和 ‘X’ 相關

2. 透過優先計算前一位與下一位是否能找到數字的規則決定結果要加上多少數字

3. 如果前一位與下一位對應的數字不在 才是累計前一位的數字

程式碼 ：

/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function(s) {
    romanDict = {
        IV: 4,
        IX: 9,
        XL: 40,
        XC: 90,
        CD: 400,
        CM: 900,
        M: 1000,
        D: 500,
        C: 100,
        L: 50,
        X: 10,
        V: 5,
        I: 1,        
    };
    
    let nextIndex = 0;
    let result = 0;
    while(nextIndex < s.length) {
        const firstChat = s[nextIndex];
        nextIndex++;
        const nextChat = s[nextIndex];
        
        if (nextChat) {
            const nextNum = romanDict[firstChat + nextChat];
            if (nextNum) {
                result += nextNum;
                nextIndex++; 
                continue;
            }
        }

        result += romanDict[firstChat];
    }
    
    return result;
};

執行成果：

高手解題：

/**
 * @param {string} s
 * @return {number}
 */
var romanToInt = function(s) {
     const a = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 };
    
    let res=0
    
    for(let i=0;i<s.length;i++){
        res+=a[s[i]]
        
        if(a[s[i-1]]<a[s[i]]){
            res-=2*a[s[i-1]]
        }
    }
    
    return res
    
};

透過先累計目前對應的數字後, 再比較是否大於前一位數字, 

如果是就代表是羅馬數字的 9 系列, 只要把前一位多加與這次要減掉的數字與結果相減後, 

就可以得到答案