Practice Problems List

Updated at September 26, 2025

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#data-structures

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Practice Problems List

Arrays — Recommended Problems for This Week - Easy (Confidence Builders)

#	Problem	Platform	Key Concept
1	Two Sum	LeetCode	HashMap lookup, complement logic
2	Contains Duplicate	LeetCode	Using `Set`
3	Find the Duplicate Number	GeeksforGeeks	In-place marking
4	Union of Two Arrays	GeeksforGeeks	HashSet union logic
5	Cyclically Rotate an Array by One	GeeksforGeeks	Rotation fundamentals
6	Sum of Array Elements	HackerRank	Simple iteration
7	Equilibrium Point	GeeksforGeeks	Prefix sums
8	Find Maximum and Minimum Element in an Array	GeeksforGeeks	Basic iteration

Medium (Pattern Recognition & Optimization) - Focus: Sliding Window, Prefix Sum, Sorting Logic, and HashMap strategies.

#	Problem	Platform	Key Concept
1	Maximum Subarray (Kadane’s Algorithm)	LeetCode	DP, prefix sums
2	3Sum	LeetCode	Two pointers
3	Product of Array Except Self	LeetCode	Prefix & suffix product
4	Subarray Sum Equals K	LeetCode	Prefix sum + HashMap
5	Sort Colors (Dutch National Flag)	LeetCode	Three pointers
6	Find All Duplicates in an Array	LeetCode	In-place modification
7	Missing Ranges	LeetCode	Edge case handling
8	Count Pairs with Given Sum	GeeksforGeeks	HashMap counting
9	Wave Array	GeeksforGeeks	Sorting & swapping pattern
10	Rotate Image (Matrix)	LeetCode	Matrix transpose + reverse rows

Advanced (Optional Stretch Goals) - Focus: Complex patterns and optimization.

#	Problem	Platform	Key Concept
1	Trapping Rain Water	LeetCode	Two-pointer prefix/suffix optimization
2	Maximum Product Subarray	LeetCode	Track min/max simultaneously
3	Longest Consecutive Sequence	LeetCode	HashSet & O(n) scan
4	First Missing Positive	LeetCode	Index marking trick
5	Spiral Matrix	LeetCode	Simulation logic
6	Merge Intervals	LeetCode	Sorting + interval merge
7	Kth Largest Element in an Array	LeetCode	Heap / QuickSelect
8	Max Chunks to Make Sorted	LeetCode	Prefix max = suffix min logic
9	Subarray with XOR = K	CodeStudio	Prefix XOR + HashMap
10	4Sum	LeetCode	Nested two-pointer technique

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Written by Rahul Aher

I'm Rahul, Sr. Software Engineer (SDE II) and passionate content creator. Sharing my expertise in software development to assist learners.