LeetCode Must-Do Lists

Curated list of high-frequency interview problems categorized by difficulty and pattern.

Easy Problems (Foundations)

Problem	Pattern	Key Concept
Roman to Integer	Hash Map	Mapping values, iteration logic
Valid Parentheses	Stack	Matching pairs, LIFO
Merge Two Sorted Lists	Linked List	Pointer manipulation
Best Time to Buy/Sell Stock	Two Pointers	Min/Max tracking
Valid Palindrome	Two Pointers	String cleanup, comparison
Invert Binary Tree	Recursion	DFS, swapping nodes
Contains Duplicate	Hash Set	O(1) lookup

Medium Problems (Core Skills)

Problem	Pattern	Key Concept
Longest Palindromic Substring	DP / Two Pointers	Expand around center
Container With Most Water	Two Pointers	Area calculation, shrinking window
3Sum	Two Pointers	Sorting, avoiding duplicates
Group Anagrams	Hash Map	Sorting strings as keys
Top K Frequent Elements	Heap / Bucket Sort	Frequency counting
Product of Array Except Self	Prefix/Suffix	Division-free calculation
Valid Sudoku	Hash Set	Row/Col/Box validation
Encode/Decode Strings	String Manipulation	Length delimiters

Hard Problems (Advanced)

Problem	Pattern	Key Concept
Median of Two Sorted Arrays	Binary Search	Partitioning arrays
Merge k Sorted Lists	Heap / Divide & Conquer	Priority Queue
Trapping Rain Water	Two Pointers / DP	Max height tracking
Regular Expression Matching	DP	Recursive matching
N-Queens	Backtracking	Constraint satisfaction

Pattern Checklist

• Sliding Window: Subarrays, substrings.
• Two Pointers: Sorted arrays, pairs.
• Fast & Slow Pointers: Cycle detection.
• Binary Search: Sorted search, rotated arrays.
• DFS/BFS: Trees, graphs, matrices.
• Backtracking: Combinations, permutations.
• Dynamic Programming: Optimization, counting.
• Intervals: Merging, inserting, overlaps.
• Tries: Prefix searches, autocomplete.
• Heap: Top K, median, scheduling.

Strategy Tips

1. Start Easy: Build confidence with Easy problems before touching Medium.
2. Timebox: Spend max 25 mins on Easy, 45 mins on Medium.
3. Speak Aloud: Explain your thought process as if in an interview.
4. Test Cases: Always check edge cases (empty input, negatives, large numbers).

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Goal: Complete the Blind 75 list first. It covers the most essential patterns needed for 90% of interviews.