DSA - 2 new (sliding window, linked list)

🧠 Today’s Problems

76. Minimum Window Substring

Insight:
Use a hashmap to track the frequency of characters in t. Expand the window until all required characters are covered. Once valid, try shrinking the window from the left to find the minimum substring.

Key ideas:

• Maintain a frequency map of t
• Use a counter to track how many characters are still needed
• Expand right pointer to satisfy condition
• Once valid → shrink from left as much as possible
• Handle edge cases where extra valid characters exist

Important Learning:
When encountering extra characters beyond required frequency, they should NOT affect the valid count. This means:

• Only decrement count when character frequency is still needed
• Only increment count back when removing a required character

Comment:
I had ~95% of the logic clear.

• Missed handling of extra characters correctly
• Didn’t implement the inner while loop for shrinking confidently
• Knew I had to shrink after valid window but couldn’t translate it into code
• Took help from Gemini to refine and complete implementation

206. Reverse Linked List

Insight:
Classic pointer manipulation problem.

• Use prev = null
• Traverse using curr = head
• Store next node temporarily
• Reverse current node’s pointer
• Move prev and curr forward

Pattern:

next = curr.next
curr.next = prev
prev = curr
curr = next

Comment:
Conceptually straightforward, but execution wasn’t as fast as expected.
Had to pause and think through pointer transitions.

⚡ Key Takeaways

• Knowing the approach ≠ being able to implement it cleanly. Improve confidence in translating thought → code
• Sliding window problems often require nested loops (expand + shrink) — don’t hesitate
• Edge cases (especially frequency handling) are where most bugs hide
• Pointer problems demand calm, step-by-step thinking, not speed

🚀 Good Momentum

• Recognizing patterns faster
• Identifying gaps in execution clearly

Feels like it’s really hard NOT to crack FAANG with a year’s grind.