【LeetCode】0013. Roman to Integer

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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, which is simply X + II. The number twenty seven 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. Input is guaranteed to be within the range from 1 to 3999.

Example 1:

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Input: "III"
Output: 3

Example 2:

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Input: "IV"
Output: 4

Example 3:

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Input: "IX"
Output: 9

Example 4:

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

Example 5:

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


Related Topics: Math、String

解題邏輯與實作

在羅馬數字的規則中有一條，若是左邊的數字比右邊小，則這個位數的值為右減左。其他情況下則需要把羅馬數至相對應的值相加。

但在實做時，若發現當前的數字比前一個數字大，才回頭更新值，會使的程式流程變得相當複雜，因此這邊將傳入的字串反過來操作。

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class Solution:
    def romanToInt(self, s: str) -> int:
         roman_to_int = {"M": 1000, "D": 500, "C": 100, "L": 50, "X": 10, "V": 5, "I": 1}
         result = 0
         pre_int = 0
         for idx , char in enumerate(s[::-1]):
             now_int = roman_to_int [char]
             result += now_int * -1 if now_int < pre_int else now_int
             pre_int = now_int
			
         return result

其他連結

  1. 【LeetCode】0000. 解題目錄