📘Day 6 – Teacher's Day Challenge | Valid Parentheses | Stack + Hash Map

🗓️ Date: July 30, 2025

📌 Challenge:
🔢 Problem: Valid Parentheses – LeetCode #20 (Easy)
🎯 Topic: Stack, Hash Map, String

💻 Platform: Leetcode

---

🔗 Table of Contents

• Problem 1 – Valid Parentheses

• Daily Summary

---

✅ Problem Statement (Summary):
Check if a given string of parentheses ()[]{} is valid. A string is valid if:

• Open brackets are closed by the same type.

• Brackets are closed in the correct order.

• Every closing bracket has a matching opening bracket.

🧾 Examples:

• "()" → true

• "()[]{}" → true

• "(]" → false

• "([])" → true

• "([)]" → false

---

🧠 What I Learned Today:

• Use a stack to keep track of open brackets.

• Use a hash map to match closing → opening brackets.

• Push when open bracket, check+pop when closing.

• Edge case: stack must be empty at end for a valid string.

---

🧪 Code (C++):

class Solution {
public:
    bool isValid(string s) {
        unordered_map<char, char> pairs = {
            {')', '('}, {']', '['}, {'}', '{'}
        };
        stack<char> st;
        for (char ch : s) {
            if (ch == '(' || ch == '{' || ch == '[') {
                st.push(ch);
            } else {
                if (st.empty() || st.top() != pairs[ch]) {
                    return false;
                }
                st.pop();
            }
        }
        return st.empty();
    }
};

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

---

📸 LeetCode Submission Screenshot:

---

✅ Key Takeaways:

• Stack is ideal for tracking nested or matching structures.

• HashMap simplifies pairing and matching logic.

• Final check on stack.empty() ensures no unmatched opens remain.

---

📈 Daily Summary

---

🗂️ Tags:
#leetcode #43DaysChallenge #stack #hashmap #dsa #cpp #validparentheses