Roman Number to Integer

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Time: O(n)

Space: O(1)

Intuition

Convert Roman numeral to integer. Scan right to left; subtract if current < next, else add.

Algorithm

1Map each symbol to value. Scan from right. If val < prevVal: subtract. Else add. Track prevVal.

Common Pitfalls

•Same as LC 13. Subtractive notation: IV=4, IX=9 etc. Right-to-left scan handles this naturally.

Roman Number to Integer.java

Java

// Approach: HashMap for values. Subtract current if next is larger, otherwise add.
// Time: O(n) Space: O(1)
import java.util.*;

class Solution {
    public int romanToDecimal(String s) {
        HashMap<Character, Integer> 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 ans = 0;
        for (int i = 0; i < s.length() - 1; i++) {
            int num1 = map.get(s.charAt(i));
            int num2 = map.get(s.charAt(i + 1));

            if (num1 >= num2)
                ans += num1;
            else
                ans -= num1;
        }
        
        return ans + map.get(s.charAt(s.length() - 1));
    }
}

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