Convert Roman Numeral to Integer (Java)

Convert Roman Numeral to Integer (Java)

In this post, we will learn how to convert a Roman numeral string into its integer value using Java. This solution efficiently handles both normal and subtractive cases using a single traversal.

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Problem Statement

Given a Roman numeral string, convert it into its corresponding integer value.

Examples:

VIII   → 8
IX     → 9
LVIII  → 58
MCMXCIV → 1994

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Approach Used

• Traverse the string from right to left
• Convert each Roman symbol to its integer value
• Add or subtract values based on Roman numeral rules

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Java Code Implementation

public class RomanToInteger {

    public static void main(String[] args) {

        String[] testCases = {
            "VIII",
            "IX",
            "LVIII",
            "MCMXCIV",
            "XL"
        };

        for (String roman : testCases) {
            System.out.println("Roman: " + roman +
                    " → Integer: " + romanToInteger(roman));
        }
    }

    private static int romanToInteger(String st) {

        int result = 0;
        int value = 0;

        for (int i = st.length() - 1; i >= 0; i--) {

            switch (st.charAt(i)) {
                case 'I': value = 1; break;
                case 'V': value = 5; break;
                case 'X': value = 10; break;
                case 'L': value = 50; break;
                case 'C': value = 100; break;
                case 'D': value = 500; break;
                case 'M': value = 1000; break;
            }

            if (value * 4 < result)
                result -= value;
            else
                result += value;
        }

        return result;
    }
}

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Explanation

Right-to-Left Traversal

• Roman numerals follow a subtractive rule (e.g., IV = 4)
• By scanning from right to left, we can decide whether to add or subtract
• If the current value is much smaller than the accumulated result, subtract it

Technique Used: Reverse Traversal + Greedy Decision

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Sample Output

Roman: VIII → Integer: 8
Roman: IX → Integer: 9
Roman: LVIII → Integer: 58
Roman: MCMXCIV → Integer: 1994
Roman: XL → Integer: 40

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Time & Space Complexity

Metric	Complexity
Time	O(n)
Space	O(1)

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Interview Tip

The right-to-left traversal avoids extra data structures and is often preferred in interviews for its simplicity and efficiency.

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Want to Go Further?

• Validate invalid Roman numerals
• Convert Integer to Roman
• Solve this using a HashMap approach

👉 Tip: This problem is commonly asked in coding interviews and tests understanding of greedy logic.

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Category: Strings
Difficulty: Easy
Techniques: Greedy, Reverse Traversal