Heap and Priority Queue: The Data Structure Behind Top-K
Learn how heaps power priority queues, why heapq runs push and pop in O(log n), and how to solve classic Top-K and merge problems in Python.
·6 min read · #dsa#heap#beginner
#dsa
277 posts · page 5 of 6
Kruskal's and Prim's MST Algorithms
Build minimum spanning trees with Kruskal's union-find approach and Prim's priority queue approach, with side-by-side Python implementations.
·5 min read · #graphs#mst#intermediate
3Sum — Sort and Two Pointers, Step by Step
A careful walkthrough of 3Sum using sort plus two pointers. We handle duplicate triplets cleanly and avoid the classic off-by-one traps.
·7 min read · #dsa#leetcode#two-pointers
Best Time to Buy and Sell Stock — One-Pass Min Tracker
Walk through 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
Binary Tree Level Order Traversal — Queue Size Snapshot
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
Climbing Stairs: Fibonacci in Disguise
Walk through 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
Clone Graph — DFS With a Node-to-Copy Hash Map
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
Coin Change: Bottom-Up DP Explained
Walk through Coin Change with brute force recursion, memoization, and bottom-up DP. Includes complexity analysis and interview script.
·6 min read · #dsa#leetcode#dynamic-programming
Container With Most Water — Two-Pointer Walkthrough
Solve Container With Most Water with the two-pointer technique. We prove correctness and contrast it with the quadratic brute force.
·6 min read · #dsa#leetcode#two-pointers
Combination Sum: Backtracking With Reuse
Solve Combination Sum with a clean backtracking template. Pruning, duplicate avoidance, complexity discussion, and interview script.
·5 min read · #dsa#leetcode#backtracking
Course Schedule — Topological Sort With DFS
Detect cycles and produce a valid course order using DFS-based topological sort. Includes BFS Kahn alternative and interview script.
·6 min read · #dsa#leetcode#graphs
Edit Distance: 2D DP Step by Step
Edit Distance fully unpacked — the three-operation recurrence, the 2D table, and the rolling-array space optimization that makes interviewers nod.
·6 min read · #dsa#leetcode#dp
Daily Temperatures — Monotonic Stack Pattern
Solve Daily Temperatures with a monotonic decreasing stack of indices. Includes brute force comparison, walkthrough, complexity, and interview tips.
·6 min read · #dsa#leetcode#stack
Group Anagrams — Hash Map of Canonical Keys
Solve the Group Anagrams problem cleanly with a hash map keyed by sorted strings or character counts. Includes complexity analysis and interview talking points.
·6 min read · #dsa#leetcode#hashing
Generate Parentheses: Counting Backtrack
Solve Generate Parentheses with counting-based backtracking. Clean invariant, walkthrough, complexity discussion, and interview tips.
·4 min read · #dsa#leetcode#backtracking
House Robber: The Pick-or-Skip DP Template
Solve 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
Invert Binary Tree — Recursive Mirror Swap
Solve 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
Kth Smallest Element in a BST — Iterative Inorder Early Exit
The inorder traversal trick, the iterative early-exit version, and the follow-up that haunts FAANG interviews.
·5 min read · #dsa#leetcode#trees
Letter Combinations of a Phone Number: Index Backtracking
Solve Letter Combinations of a Phone Number with backtracking. Mapping setup, recursive enumeration, complexity, and interview walkthrough.
·5 min read · #dsa#leetcode#backtracking
Longest Consecutive Sequence — The Hash Set Trick
Solve Longest Consecutive Sequence in O(n) using a hash set and a start-of-run check. Walkthrough, edge cases, and interview script.
·5 min read · #dsa#leetcode#hashing
Longest Increasing Subsequence: DP and Patience
Longest Increasing Subsequence in detail — the O(n^2) DP, the O(n log n) patience-sorting trick with binary search, and when each one matters.
·6 min read · #dsa#leetcode#dp
Longest Palindromic Substring — Expand Around Center
Walk through the Longest Palindromic Substring problem using the expand-around-center technique. Compare brute force, DP, and the optimal approach with examples.
·7 min read · #dsa#leetcode#strings
LRU Cache — Hash Map + Doubly Linked List
Design LRU Cache with a hash map and a doubly linked list to get O(1) get and put, with a clean implementation and interview tips.
·6 min read · #dsa#leetcode#design
Longest Substring Without Repeating Characters — Sliding Window
Solve 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
Lowest Common Ancestor of a BST — One-Line Guided Walk
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
Maximum Depth of Binary Tree — DFS in Three Lines
Compute tree depth with a three-line recursive DFS and an iterative BFS alternative, with complexity analysis and interview tips.
·5 min read · #dsa#leetcode#binary-tree
Maximum Subarray — Kadane's Algorithm Explained
A clear walkthrough of 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
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.
·5 min read · #dsa#leetcode#stack
Number of Islands — Grid DFS Flood Fill
Solve 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
Merge Two Sorted Lists — The Dummy-Head Pattern
Solve 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
Pacific Atlantic Water Flow — Flood Inward From the Borders
Why reverse traversal beats per-cell flood fill, and how the intersection of two reach sets gives the answer.
·7 min read · #dsa#leetcode#graphs
Path Sum — DFS With a Running Target
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
Permutations: Backtracking With Used Array
Solve Permutations with backtracking and a used array. Compare swap-in-place vs used-array, walkthrough, complexity, and interview tips.
·5 min read · #dsa#leetcode#backtracking
Product of Array Except Self — Prefix and Suffix, No Division
Solve Product of Array Except Self in O(n) without division using prefix and suffix passes. Clean walkthrough plus interview script.
·5 min read · #dsa#leetcode#arrays
Reverse Linked List — Iterative and Recursive
Walk through 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
Rotate Image — Transpose Plus Row Reverse
Rotate an n by n matrix 90 degrees in place by transposing and reversing each row. Walkthrough, edge cases, complexity, and interview script.
·4 min read · #dsa#leetcode#arrays
Search in Rotated Sorted Array: Modified Binary Search
Solve Search in Rotated Sorted Array in O(log n) with modified binary search. Pivot detection, half-decision logic, and interview talking points.
·5 min read · #dsa#leetcode#binary-search
Serialize and Deserialize Binary Tree — Preorder With Null Sentinels
The preorder-with-nulls encoding, the iterator-driven decoder, and why level-order is a worse choice than it looks.
·6 min read · #dsa#leetcode#trees
Sliding Window Maximum — Monotonic Deque Pattern
Solve Sliding Window Maximum in O(n) using a monotonic deque. Step-by-step walkthrough, interview script, and complexity analysis.
·5 min read · #dsa#leetcode#sliding-window
Spiral Matrix — Four-Boundary Walk Template
Solve Spiral Matrix with the four-boundary walk pattern. Clean implementation, edge cases for rectangles, complexity, and interview tips.
·5 min read · #dsa#leetcode#arrays
Subsets: The Backtracking Template Every Interview Tests
Generate all subsets of an array using backtracking and iterative bit-mask approaches. Includes complexity analysis and interview script.
·6 min read · #dsa#leetcode#backtracking
Top K Frequent Elements — Heap vs Bucket Sort
Solve Top K Frequent Elements with both a min-heap and a bucket sort approach. Trade-offs, complexity, and interview-ready walkthrough.
·5 min read · #dsa#leetcode#hashing
Two Sum — From Brute Force to O(n) Hash Map
A complete walkthrough of the Two Sum problem. We move from the obvious nested loop to a single-pass hash map and dissect why it works.
·6 min read · #dsa#leetcode#hashing
Trapping Rain Water — Two-Pointer in O(n)
Solve Trapping Rain Water in linear time and constant space using the two-pointer technique. Brute force, optimal walkthrough, and interview talk track.
·5 min read · #dsa#leetcode#two-pointers
Unique Paths: Grid DP and Math
Unique Paths 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
Valid Anagram — Counting Characters in O(n)
A complete walkthrough of the Valid Anagram problem. 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
Valid Parentheses — The Classic Stack Pattern
A clean walkthrough of Valid Parentheses with the optimal stack solution. Covers edge cases, complexity, and how to explain the approach in interviews.
·6 min read · #dsa#leetcode#stack
Validate Binary Search Tree — Min/Max Bounds DFS
The seductive wrong answer, the correct min/max bounds approach, and how to defend it in an interview.
·5 min read · #dsa#leetcode#trees