Roman To Integer [Code + Interactive Visualization]

String Easy

Python Java C++

def romanToInt(s: str) -> int:
    # Handle edge cases
    if not s: return -1
    
    # Map roman numerals to values
    map = {
        'I': 1,
        'V': 5,
        'X': 10,
        'L': 50,
        'C': 100,
        'D': 500,
        'M': 1000
    }
    
    n = len(s)
    # Initialize with last character's value
    result = map[s[n - 1]]
    
    # Process remaining characters right to left
    for i in range(n - 2, -1, -1):
        val = map[s[i]]
        next = map[s[i + 1]]
        
        if val >= next: result += val
        if val < next: result -= val
            
    return result

class Solution {
    public int romanToInt(String s) {
        
         if (s == null || s.length() == 0) return -1;
        HashMap map = new HashMap();
        map.put('I',1);
        map.put('V',5);
        map.put('X',10);
        map.put('L',50);
        map.put('C',100);
        map.put('D',500);
        map.put('M',1000);
        
        int n = s.length();
        
        int result = map.get(s.charAt(n - 1));
        
        for (int i = 0; i <= n - 2; i++) {
            
            int val = map.get(s.charAt(i));
            int next = map.get(s.charAt(i + 1));
            
            if (val >= next) result += val;
            if (val < next) result -= val;
            
            
        }
        
        return result;
    }
}

class Solution {
public:
    int romanToInt(string s) {
        // Handle edge cases
        if (s.empty()) return -1;
        
        // Map roman numerals to values
        unordered_map map = {
            {'I', 1},
            {'V', 5},
            {'X', 10},
            {'L', 50},
            {'C', 100},
            {'D', 500},
            {'M', 1000}
        };
        
        int n = s.length();
        int result = map[s[n - 1]];
        
        // Process remaining characters right to left
        for (int i = n - 2; i >= 0; i--) {
            int val = map[s[i]];
            int next = map[s[i + 1]];
            
            if (val >= next) result += val;
            if (val < next) result -= val;
        }
        
        return result;
    }
};

Problem Statement

Given a string s containing valid Roman numerals, convert it to an integer. Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Detailed Explanation

Approach

The solution processes the Roman numeral string by comparing each character with the next one. If a smaller value precedes a larger value, we subtract it; otherwise, we add it. We start from the rightmost character and work our way left.

Key Concepts

1. Roman Numeral Rules: When a smaller value appears before a larger value, it represents subtraction
2. Hash Map Usage: Quick lookup of Roman numeral values
3. Right to Left Processing: Compare current value with the next value to determine addition or subtraction

Algorithm Steps

1. Create a hash map of Roman numeral values
2. Get the value of the last character as initial result
3. For remaining characters from right to left:
   • Compare current value with next value
   • If current ≥ next: Add current value
   • If current < next: Subtract current value
4. Return final result

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