DSA Problems

Interview problems worked from brute force to optimal, each with the pattern signal that identifies it, C++ and JavaScript solutions, complexity analysis, and a step-by-step visualizer.

Problem	Difficulty	Pattern	Week
3Sum	medium	two-pointers	1
Container With Most Water	medium	two-pointers	1
Contains Duplicate	easy	frequency-counting	1
Encode and Decode Strings	medium	string	1
Group Anagrams	medium	frequency-counting	1
Longest Consecutive Sequence	medium	frequency-counting	1
Product of Array Except Self	medium	prefix-sum	1
Top K Frequent Elements	medium	frequency-counting	1
Trapping Rain Water	hard	two-pointers	1
Two Sum	easy	frequency-counting	1
Two Sum II — Input Array Is Sorted	medium	two-pointers	1
Valid Anagram	easy	frequency-counting	1
Valid Palindrome	easy	two-pointers	1
Valid Sudoku	medium	frequency-counting	1
Best Time to Buy and Sell Stock	easy	sliding-window-fixed	2
Longest Repeating Character Replacement	medium	sliding-window-variable	2
Longest Substring Without Repeating Characters	medium	sliding-window-variable	2
Maximum Average Subarray I	easy	sliding-window-fixed	2
Minimum Window Substring	hard	sliding-window-variable	2
Permutation in String	medium	sliding-window-fixed	2
Sliding Window Maximum	hard	sliding-window-fixed	2
Subarray Sum Equals K	medium	prefix-sum	2
Car Fleet	medium	monotonic-stack, sorting	3
Daily Temperatures	medium	monotonic-stack	3
Evaluate Reverse Polish Notation	medium	stack	3
Largest Rectangle in Histogram	hard	monotonic-stack	3
Min Stack	medium	stack, design	3
Valid Parentheses	easy	stack	3
Binary Search	easy	binary-search	4
Find Minimum in Rotated Sorted Array	medium	binary-search	4
Koko Eating Bananas	medium	binary-search-on-answer	4
Median of Two Sorted Arrays	hard	binary-search	4
Search a 2D Matrix	medium	binary-search	4
Search in Rotated Sorted Array	medium	binary-search	4
Time Based Key-Value Store	medium	binary-search	4
Add Two Numbers	medium	dummy-head, carry-simulation	5
Copy List with Random Pointer	medium	interleaving, deep-copy	5
Find the Duplicate Number	medium	fast-and-slow-pointers, implicit-graph	5
Linked List Cycle	easy	fast-and-slow-pointers, floyd-cycle-detection	5
LRU Cache	medium	design, hash-map, doubly-linked-list	5
Merge K Sorted Lists	hard	k-way-merge, min-heap, dummy-head	5
Merge Two Sorted Lists	easy	dummy-head, two-pointers	5
Remove Nth Node From End	medium	fast-and-slow-pointers, dummy-head	5
Reorder List	medium	fast-and-slow-pointers, in-place-reversal, list-weaving	5
Reverse Linked List	easy	in-place-reversal	5
Reverse Nodes in k-Group	hard	in-place-reversal, dummy-head, group-processing	5