Roman to Integer

Leetcode • Question List

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].

Solution

In my solution I create two maps, one called solution_map and the other called exception_map. In the solution_map, we map each roman symbol with it's value.

Three of the characters (I,X, and C) behave differently if they are placed before their exception symbols. For all three characters there are only 2 exceptions, so I map the characters to their exceptions as a combined string.

1unordered_map<char, int> solution_map {
2    {'I', 1},
3    {'V', 5},
4    {'X', 10},
5    {'L', 50},
6    {'C', 100},
7    {'D', 500},
8    {'M', 1000}
9};
10unordered_map<char, string> exception_map {
11    {'I', "VX"},
12    {'X', "LC"},
13    {'C', "DM"}
14};

For my main logic, I have it check if the character is one of the exception symbols. If it is, we first check if it's at the end of the string or not to avoid causing an out of range error.

If it's not the end of the string, we check if the next character is one of their exception symbols. If it is we subtract instead of adding.

In all other cases we add the value.