LeetCode 3Sum: Sort + Two Pointers Step by Step
A careful walkthrough of LeetCode 15 3Sum using sort plus two pointers. We handle duplicate triplets cleanly and avoid the classic off-by-one traps.
·6 min read · #dsa#leetcode#two-pointers
#leetcode
48 posts.
LeetCode Best Time to Buy and Sell Stock: One-Pass O(n)
Walk through LeetCode 121 Best Time to Buy and Sell Stock. We go from the quadratic brute force to a clean one-pass solution tracking the running minimum.
·5 min read · #dsa#leetcode#arrays
LeetCode Climbing Stairs: Fibonacci in Disguise
Walk through LeetCode 70 Climbing Stairs from brute force recursion to bottom-up DP, with edge cases, complexity analysis, and an interview script.
·5 min read · #dsa#leetcode#dynamic-programming
LeetCode Coin Change: Bottom-Up DP Explained
Walk through LeetCode 322 Coin Change with brute force recursion, memoization, and bottom-up DP. Includes complexity analysis and interview script.
·6 min read · #dsa#leetcode#dynamic-programming
LeetCode Container With Most Water: Two-Pointer Walkthrough
Solve LeetCode 11 Container With Most Water with the two-pointer technique. We prove correctness and contrast it with the quadratic brute force.
·5 min read · #dsa#leetcode#two-pointers
LeetCode Course Schedule: Topological Sort with DFS
Detect cycles and produce a valid course order in LeetCode 207 Course Schedule using DFS-based topological sort. Includes BFS Kahn alternative and interview script.
·6 min read · #dsa#leetcode#graphs
LeetCode Daily Temperatures: Monotonic Stack Pattern
Solve LeetCode Daily Temperatures with a monotonic decreasing stack of indices. Includes brute force comparison, walkthrough, complexity, and interview tips.
·6 min read · #dsa#leetcode#stack
LeetCode Group Anagrams: Hash Map of Sorted Keys
Solve LeetCode Group Anagrams cleanly with a hash map keyed by sorted strings or character counts. Includes complexity analysis and interview talking points.
·5 min read · #dsa#leetcode#hashing
LeetCode House Robber: The Pick-or-Skip DP Template
Solve LeetCode 198 House Robber with the pick-or-skip DP recurrence, then optimize to O(1) space. Includes interview script and related variants.
·5 min read · #dsa#leetcode#dynamic-programming
LeetCode Invert Binary Tree: 4-Line Recursion
Solve LeetCode 226 Invert Binary Tree with a four-line recursive swap and an iterative BFS alternative, including complexity and interview talking points.
·5 min read · #dsa#leetcode#binary-tree
LeetCode Longest Palindromic Substring: Expand Around Center
Walk through LeetCode Longest Palindromic Substring using the expand-around-center technique. Compare brute force, DP, and the optimal approach with examples.
·6 min read · #dsa#leetcode#strings
LeetCode Longest Substring Without Repeating Characters
Solve LeetCode 3 Longest Substring Without Repeating Characters using a sliding window with a hash map. We go from brute force to a clean O(n) sweep.
·5 min read · #dsa#leetcode#sliding-window
LeetCode LRU Cache: Hash Map + Doubly Linked List
Design LeetCode 146 LRU Cache with a hash map and a doubly linked list to get O(1) get and put, with a clean Python implementation and interview tips.
·5 min read · #dsa#leetcode#design
LeetCode Maximum Depth of Binary Tree: DFS in 3 Lines
Solve LeetCode 104 Maximum Depth of Binary Tree with a three-line recursive DFS and an iterative BFS alternative, with complexity analysis and interview tips.
·5 min read · #dsa#leetcode#binary-tree
LeetCode Maximum Subarray: Kadane's Algorithm Explained
A clear walkthrough of LeetCode 53 Maximum Subarray. We build Kadane's algorithm from first principles and contrast it with the divide and conquer approach.
·5 min read · #dsa#leetcode#arrays
LeetCode Merge Two Sorted Lists: The Dummy-Head Pattern
Solve LeetCode 21 Merge Two Sorted Lists with the dummy-head pointer pattern, plus a recursive variant, edge cases, and interview explanation tips.
·5 min read · #dsa#leetcode#linked-list
LeetCode Min Stack: Track Mins in O(1)
Design a stack that supports push, pop, top, and getMin in constant time. Walkthrough of the two-stack and pair-stack solutions with edge cases and interview tips.
·6 min read · #dsa#leetcode#stack
LeetCode Number of Islands: Grid DFS Step by Step
Solve LeetCode 200 Number of Islands with grid DFS and BFS, including a union-find variant, edge-case handling, and clear interview talking points.
·6 min read · #dsa#leetcode#graph
LeetCode Reverse Linked List: Iterative and Recursive
Walk through LeetCode 206 Reverse Linked List with iterative pointer flipping and a clean recursive solution, plus complexity and interview talking points.
·5 min read · #dsa#leetcode#linked-list
LeetCode Subsets: The Backtracking Template Every Interview Tests
Generate all subsets of an array in LeetCode 78 using backtracking and iterative bit-mask approaches. Includes complexity analysis and interview script.
·6 min read · #dsa#leetcode#backtracking
LeetCode Two Sum: Brute Force to O(n) Hash Map
A complete walkthrough of LeetCode 1 Two Sum. We move from the obvious nested loop to a single-pass hash map and dissect why it works.
·5 min read · #dsa#leetcode#hashing
LeetCode Valid Parentheses: The Classic Stack Problem
A clean walkthrough of LeetCode Valid Parentheses with the optimal stack solution. Covers edge cases, complexity, and how to explain the approach in interviews.
·5 min read · #dsa#leetcode#stack
LeetCode Valid Anagram: Counting Characters in O(n)
A complete walkthrough of LeetCode Valid Anagram. Compare the sorting approach with the optimal hash map counting solution and learn how to explain it in interviews.
·6 min read · #dsa#leetcode#hashing
LeetCode Word Break: DP with a Word Dictionary
Solve LeetCode 139 Word Break with bottom-up dynamic programming. Includes brute force, edge cases, complexity analysis, and an interview script.
·6 min read · #dsa#leetcode#dynamic-programming
LeetCode Binary Tree Level Order Traversal
LeetCode 102 in detail — the queue-with-size BFS pattern, why DFS still works, and how this template extends to zigzag and right-side view.
·5 min read · #dsa#leetcode#trees
LeetCode Clone Graph: DFS with a Hash Map
LeetCode 133 walked through — why a hash map from original to copy is the entire idea, plus the BFS variant for stack-limited environments.
·5 min read · #dsa#leetcode#graphs
LeetCode Combination Sum: Backtracking Template
Solve LeetCode 39 Combination Sum with a clean backtracking template. Pruning, duplicate avoidance, complexity discussion, and interview script.
·3 min read · #dsa#leetcode#backtracking
LeetCode Edit Distance: 2D DP Step by Step
LeetCode 72 fully unpacked — the three-operation recurrence, the 2D table, and the rolling-array space optimization that makes interviewers nod.
·5 min read · #dsa#leetcode#dp
LeetCode Generate Parentheses: Counting Backtrack
Solve LeetCode 22 Generate Parentheses with counting-based backtracking. Clean invariant, walkthrough, complexity discussion, and interview tips.
·3 min read · #dsa#leetcode#backtracking
LeetCode Kth Smallest Element in a BST
LeetCode 230 in detail — the inorder traversal trick, the iterative early-exit version, and the follow-up that haunts FAANG interviews.
·4 min read · #dsa#leetcode#trees
LeetCode Letter Combinations of a Phone Number
Solve LeetCode 17 Letter Combinations of a Phone Number with backtracking. Mapping setup, recursive enumeration, complexity, and interview walkthrough.
·3 min read · #dsa#leetcode#backtracking
LeetCode Longest Consecutive Sequence: Hash Set Trick
Solve LeetCode 128 Longest Consecutive Sequence in O(n) using a hash set and a start-of-run check. Walkthrough, edge cases, and interview script.
·4 min read · #dsa#leetcode#hashing
LeetCode Longest Increasing Subsequence: DP and Patience
LeetCode 300 in detail — the O(n^2) DP, the O(n log n) patience-sorting trick with binary search, and when each one matters.
·5 min read · #dsa#leetcode#dp
LeetCode Lowest Common Ancestor of a BST
LeetCode 235 walked through end to end — why BST ordering collapses LCA into a one-line traversal, and the iterative version that needs zero extra space.
·5 min read · #dsa#leetcode#trees
LeetCode Pacific Atlantic Water Flow
LeetCode 417 explained — why you should flood from the oceans inward, not from each cell outward, and how the intersection gives the answer.
·6 min read · #dsa#leetcode#graphs
LeetCode Path Sum: DFS With a Running Total
LeetCode 112 in detail — the DFS-with-decrement trick, why the leaf condition is the whole problem, and the variants that build on it.
·5 min read · #dsa#leetcode#trees
LeetCode Permutations: Backtracking With Used Array
Solve LeetCode 46 Permutations with backtracking and a used array. Compare swap-in-place vs used-array, walkthrough, complexity, and interview tips.
·3 min read · #dsa#leetcode#backtracking
LeetCode Product of Array Except Self: No Division
Solve LeetCode 238 Product of Array Except Self in O(n) without division using prefix and suffix passes. Clean walkthrough plus interview script.
·4 min read · #dsa#leetcode#arrays
LeetCode Rotate Image: Transpose Plus Reverse
Solve LeetCode 48 Rotate Image in place by transposing and reversing each row. Walkthrough, edge cases, complexity, and interview script.
·3 min read · #dsa#leetcode#arrays
LeetCode Search in Rotated Sorted Array
Solve LeetCode 33 Search in Rotated Sorted Array in O(log n) with modified binary search. Pivot detection, half-decision logic, and interview talking points.
·4 min read · #dsa#leetcode#binary-search
LeetCode Serialize and Deserialize Binary Tree
LeetCode 297 fully unpacked — the preorder-with-nulls encoding, the iterator-driven decoder, and why level-order is a worse choice than it looks.
·5 min read · #dsa#leetcode#trees
LeetCode Sliding Window Maximum: Deque Pattern
Solve LeetCode 239 Sliding Window Maximum in O(n) using a monotonic deque. Step-by-step walkthrough, interview script, and complexity analysis.
·4 min read · #dsa#leetcode#sliding-window
LeetCode Spiral Matrix: Boundary Walk Pattern
Solve LeetCode 54 Spiral Matrix with the four-boundary walk pattern. Clean implementation, edge cases for rectangles, complexity, and interview tips.
·4 min read · #dsa#leetcode#arrays
LeetCode Top K Frequent Elements: Heap and Bucket Sort
Solve LeetCode 347 Top K Frequent Elements with both a min-heap and a bucket sort approach. Trade-offs, complexity, and interview-ready walkthrough.
·4 min read · #dsa#leetcode#hashing
LeetCode Trapping Rain Water: Two-Pointer in O(n)
Solve LeetCode 42 Trapping Rain Water in linear time and constant space using the two-pointer technique. Brute force, optimal walkthrough, and interview talk track.
·4 min read · #dsa#leetcode#two-pointers
LeetCode Unique Paths: Grid DP and Math
LeetCode 62 in two ways — the O(m * n) grid DP that interviewers expect and the binomial-coefficient closed form that surprises them.
·5 min read · #dsa#leetcode#dp
LeetCode Validate Binary Search Tree
LeetCode 98 in full — the seductive wrong answer, the correct min/max bounds approach, and how to defend it in an interview.
·5 min read · #dsa#leetcode#trees
LeetCode Word Ladder: BFS on Word Graphs
LeetCode 127 walked through — the wildcard-pattern adjacency trick, why BFS is mandatory, and the bidirectional speedup.
·5 min read · #dsa#leetcode#graphs