📘 Day 5 – Teacher's Day Challenge | Roman to Integer | Hash Map + Subtraction Rule

🗓️ Date: July 29, 2025

📌 Challenge:

🔢 Problem: Roman to Integer – LeetCode #13 (Easy)
🎯 Topic: Strings, Hash Map, Maths

💻 Platform: Leetcode

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🔗 Table of Contents

• Problem 1 – Roman to Integer

• Daily Summary

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✅ Problem Statement (Summary):

Convert a Roman numeral string to its corresponding integer.

Roman symbols and values:

Symbol  Value
I       1  
V       5  
X       10  
L       50  
C       100  
D       500  
M       1000

Subtraction cases:

• I before V or X → 4, 9

• X before L or C → 40, 90

• C before D or M → 400, 900

🧾 Example:

• "III" → 3

• "LVIII" → 58

• "MCMXCIV" → 1994

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🧠 What I Learned Today:

• Use a hash map for fast Roman symbol lookup.

• Key logic: if current symbol value is less than the next, subtract it. Otherwise, add it.

• This approach simplifies subtraction rules elegantly in a single loop.

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🧪 Code (C++):

class Solution {
public:
    int romanToInt(string s) {
        unordered_map<char, int> roman = {{'I', 1},   {'V', 5},   {'X', 10},
                                          {'L', 50},  {'C', 100}, {'D', 500},
                                          {'M', 1000}};

        int result = 0;
        for (int i = 0; i < s.length(); i++) {
            int curr = roman[s[i]];
            int next = (i + 1 < s.length()) ? roman[s[i + 1]] : 0;

            if (curr < next) {
                result -= curr;
            } else {
                result += curr;
            }
        }
        return result;
    }
};

🕒 Time Complexity: O(n)
📦 Space Complexity: O(1)

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📸 LeetCode Submission Screenshot:

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✅ Key Takeaways:

• Subtractive notation is the only tricky part (like IV, IX, CM).

• HashMap gives O(1) lookup for Roman values.

• Always check the next character during traversal.

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📈 Daily Summary:

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🗂️ Tags:

#leetcode #43DaysChallenge #string #hashmap #dsa #cpp #codingjourney