13. Roman to Integer

Problem 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 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.

Examples

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.

class Solution:
    def romanToInt(self, s: str) -> int:
        roman_values = {
            'I': 1, 'V': 5, 'X': 10, 'L': 50,
            'C': 100, 'D': 500, 'M': 1000
        }
        
        result = 0
        prev_value = 0
        
        for char in reversed(s):
            curr_value = roman_values[char]
            if curr_value >= prev_value:
                result += curr_value
            else:
                result -= curr_value
            prev_value = curr_value
            
        return result

class Solution {
    public int romanToInt(String s) {
        Map romanValues = new HashMap<>();
        romanValues.put('I', 1);
        romanValues.put('V', 5);
        romanValues.put('X', 10);
        romanValues.put('L', 50);
        romanValues.put('C', 100);
        romanValues.put('D', 500);
        romanValues.put('M', 1000);
        
        int result = 0;
        int prevValue = 0;
        
        for (int i = s.length() - 1; i >= 0; i--) {
            int currValue = romanValues.get(s.charAt(i));
            if (currValue >= prevValue) {
                result += currValue;
            } else {
                result -= currValue;
            }
            prevValue = currValue;
        }
        
        return result;
    }
}

class Solution {
public:
    int romanToInt(string s) {
        unordered_map romanValues = {
            {'I', 1}, {'V', 5}, {'X', 10}, {'L', 50},
            {'C', 100}, {'D', 500}, {'M', 1000}
        };
        
        int result = 0;
        int prevValue = 0;
        
        for (int i = s.length() - 1; i >= 0; i--) {
            int currValue = romanValues[s[i]];
            if (currValue >= prevValue) {
                result += currValue;
            } else {
                result -= currValue;
            }
            prevValue = currValue;
        }
        
        return result;
    }
};

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

public class Solution {
    public int RomanToInt(string s) {
        var romanValues = new Dictionary {
            {'I', 1}, {'V', 5}, {'X', 10}, {'L', 50},
            {'C', 100}, {'D', 500}, {'M', 1000}
        };
        
        int result = 0;
        int prevValue = 0;
        
        for (int i = s.Length - 1; i >= 0; i--) {
            int currValue = romanValues[s[i]];
            if (currValue >= prevValue) {
                result += currValue;
            } else {
                result -= currValue;
            }
            prevValue = currValue;
        }
        
        return result;
    }
}