Day 6: Cracking My First Hard Problem & Leveling Up with Stacks, Two Pointers & Sliding Window 🚀

📌 Today’s Progress

• First time facing a hard problem—and honestly, after solving it, the solution felt much simpler than expected.

• Key realization: Understanding the question before attempting to solve it makes a huge difference.

• Covered Stack, Two Pointers, and Sliding Window problems today.

• Took quite a few breaks, but overall, it was a pretty productive day!

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📝 Problems Solved Today

Stack

1️⃣ Daily Temperatures (Medium)
💡 Concept: Monotonic Stack
🔍 Approach:

• Used a decreasing stack to track temperatures and find how many days until a warmer temperature.

• If the current temperature is higher than the top of the stack, pop elements and calculate the difference.
  ⏳ Time Complexity: O(n)

2️⃣ Car Fleet (Medium)
💡 Concept: Stack + Sorting
🔍 Approach:

• Sorted cars by position and iterated from the back, maintaining a stack to track the fleets.

• If a car reaches the destination after another, it forms a new fleet.
  ⏳ Time Complexity: O(n log n)

3️⃣ Largest Rectangle in Histogram (Hard)
💡 Concept: Monotonic Stack
🔍 Approach:

• Used a stack to track bar heights, calculating max areas whenever we encountered a smaller bar.

• Popped elements from the stack to determine the widest possible rectangle.
  ⏳ Time Complexity: O(n)

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Two Pointers

4️⃣ Valid Palindrome (Easy)
💡 Concept: Two Pointers
🔍 Approach:

• Filtered out non-alphanumeric characters and used two pointers to compare characters from both ends.
  ⏳ Time Complexity: O(n)

5️⃣ Two Sum II - Sorted Array (Medium)
💡 Concept: Two Pointers
🔍 Approach:

• Used two pointers, starting at both ends, moving them closer based on sum comparison with the target.
  ⏳ Time Complexity: O(n)

6️⃣ 3Sum (Medium)
💡 Concept: Sorting + Two Pointers
🔍 Approach:

• Sorted the array and used a fixed pointer with two moving pointers to find unique triplets summing to zero.
  ⏳ Time Complexity: O(n²)

7️⃣ Container With Most Water (Medium)
💡 Concept: Two Pointers
🔍 Approach:

• Used two pointers moving inward to maximize water storage while maintaining the widest possible width.
  ⏳ Time Complexity: O(n)

8️⃣ Trapping Rain Water (Hard)
💡 Concept: Two Pointers
🔍 Approach:

• Maintained two pointers tracking the max left and right boundaries to calculate trapped water efficiently.
  ⏳ Time Complexity: O(n)

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Sliding Window

9️⃣ Best Time to Buy and Sell Stock (Easy)
💡 Concept: Sliding Window (Optimized as a One-Pass Solution)
🔍 Approach:

• Used a single pass to track the minimum price so far and update the maximum profit dynamically.

• The key was to keep an eye on the lowest buying price and sell whenever we found a higher price that increased profit.
  ⏳ Time Complexity: O(n)

🔟 Longest Substring Without Repeating Characters (Medium)
💡 Concept: Sliding Window + HashSet
🔍 Approach:

• Used a moving window and a HashSet to track unique characters.

• If a duplicate was found, moved the left pointer until uniqueness was restored.
  ⏳ Time Complexity: O(n)

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🚀 Takeaways

🔹 Tackling a hard problem gave a huge confidence boost. It was intimidating at first, but once broken down, the logic clicked.
🔹 Recognizing problem patterns (stacks, two pointers, sliding window) makes solving easier.
🔹 Breaking down the problem before jumping into code is key. Saves time and avoids confusion.

Even with a few breaks, today was a solid grind! On to the next one. 🔥