LeetCode 678: Valid Parenthesis String ✅ Greedy + DP + Stack Approaches | MAANG Interview Prep

LeetCode 678: Valid Parenthesis String ✅ Greedy + DP + Stack Approaches | MAANG Interview Prep In this video, we’ll solve LeetCode 678 – Valid Parenthesis String, a tricky but beautiful problem where the input string may contain '(', ')', and '*' — and we need to check if it can form a valid parenthesis sequence. This is a high-frequency MAANG interview question that tests your ability to combine Greedy, Dynamic Programming, and Stack techniques. Companies like Google, Amazon, Meta, Microsoft, and Apple have asked variations of this problem in real interviews. What you’ll learn in this video: 📖 Problem breakdown with examples ❌ Why naive brute force recursion explodes (2^N possibilities) ⚡ Greedy Approach (tracking min & max open brackets) 🏗️ Stack-based solution for intuition building 🎯 Dynamic Programming solution (for completeness) 🐞 Edge cases: empty string, all *, nested/unbalanced sequences 🏆 Final O(N) Greedy Solution that interviewers expect Timestamps 00:00 Problem introduction with examples 03:20 Brute force recursion & why it fails 06:40 Stack-based approach explained 10:00 Dynamic Programming approach 13:45 Optimal Greedy approach (min-max balance) 19:00 Code walkthrough (clean + interview-ready) 23:00 Edge cases & pitfalls 26:00 Time & Space complexity analysis 28:00 Interview follow-up questions 📌 This is part of our Interview Stack & Parenthesis Problems Series, following LeetCode 84 (Largest Rectangle in Histogram) and LeetCode 402 (Remove K Digits). Mastering this will make you rock-solid in Greedy + Stack patterns that repeat across MAANG interviews. 🔥 If you’re preparing for MAANG/MANG roles, this problem is a must-know pattern. 👉 Like, Subscribe & Share to support the channel — let’s aim for 1 Million views together! 💪