LeetCode Log

A public interview-prep journal for turning LeetCode practice into reusable patterns, not just final answers.

Each problem entry includes the final Python solution, complexity analysis, pitfalls, breakthroughs, and a short mental model to make the pattern easier to recognize in interviews.

Current Entries

Problem	Difficulty	Pattern
Two Sum	Easy	Hash map lookup
Contains Duplicate	Easy	Hash set membership
Valid Anagram	Easy	Frequency counting
Valid Palindrome	Easy	Opposite-end two pointers
Remove Duplicates from Sorted Array	Easy	Two pointers / in-place array
Find the Index of the First Occurrence in a String	Easy	Fixed-size sliding window
Linked List Cycle	Easy	Fast/slow pointers
Reverse Linked List	Easy	Linked list pointer reversal
Merge Two Sorted Lists	Easy	Dummy node / tail pointer
Best Time to Buy and Sell Stock	Easy	Running minimum / max profit
Binary Search	Easy	Binary search
Group Anagrams	Medium	Frequency signature grouping
Two Sum II - Input Array Is Sorted	Medium	Opposite-end two pointers
Container With Most Water	Medium	Opposite-end two pointers
3Sum	Medium	Sorting / two pointers
Top K Frequent Elements	Medium	Frequency map / heap
Product of Array Except Self	Medium	Prefix/suffix products
Longest Substring Without Repeating Characters	Medium	Variable sliding window
Permutation in String	Medium	Fixed-size sliding window
Longest Repeating Character Replacement	Medium	Variable sliding window
Rotate Image	Medium	Matrix transforms
Valid Sudoku	Medium	Constraint tracking with sets
Longest Consecutive Sequence	Medium	Hash set / sequence starts
Encode and Decode Strings	Medium	Length-prefix encoding
Pow(x, n)	Medium	Divide and conquer
Reorder List	Medium	Linked lists / two pointers
Remove Nth Node From End of List	Medium	Linked list fast/slow pointers
Search a 2D Matrix	Medium	Binary search
Course Schedule	Medium	Graph DFS / cycle detection
Course Schedule II	Medium	Graph DFS / topological ordering
Walls and Gates	Medium	Multi-source grid BFS
Word Ladder	Hard	BFS / implicit graph
Trapping Rain Water	Hard	Two pointers / running maximums
Minimum Window Substring	Hard	Variable sliding window
Sliding Window Maximum	Hard	Monotonic deque
Implement Trie (Prefix Tree)	Medium	Trie / prefix tree
Find the Duplicate Number	Medium	Floyd's cycle detection
Car Fleet	Medium	Ordered sweep / monotonic state
Best Time to Buy and Sell Stock with Cooldown	Medium	DP / memoized state search

What This Shows

• Clear problem decomposition under interview constraints.
• Python implementations with time and space complexity.
• Notes on mistakes and boundary cases, not only polished final code.
• Small commits that show steady practice and reflection.

Workflow

1. Create a new problem folder.
2. Start your timer.
3. Work in the problem folder and leave notes as you go.
4. Commit when you finish or hit a useful stopping point.

Create A Problem Entry

python3 scripts/new_problem.py "Two Sum" easy

This creates a folder under problems/ with:

• README.md for timing, notes, insights, and reflection.
• solution.py for the main solution.
• alternatives.md for other approaches you try later.

Commit Style

Use small commits so your GitHub activity tells the story:

git add .
git commit -m "Solve two sum"

For partial progress:

git commit -m "Work through two sum brute force"