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Stacks & Queues — Complete Guide: All Patterns & Problem Index
Master every stack and queue pattern: monotonic stack, BFS, two-stack tricks, deque, and design — with templates and full index of 50 problems.
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Queue
50 articles
dsa2 min read
Valid Parentheses — Stack Bracket Matching
Check if a string of brackets is valid by pushing open brackets and matching close brackets against the stack.
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Implement Stack Using Queues — Single Queue Rotation
Implement a stack using one queue by rotating all elements after each push to bring the new element to the front.
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Implement Queue Using Stacks — Amortized O(1) Two-Stack
Implement a queue using two stacks with amortized O(1) operations by lazily transferring from inbox to outbox.
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Min Stack — Parallel Min Tracking Stack
Design a stack with O(1) getMin by maintaining a parallel min-tracking stack alongside the main stack.
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Baseball Game — Stack Simulation
Simulate a baseball scoring game using a stack to track valid scores and handle +, D, and C operations.
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Backspace String Compare — Two Pointer O(1) Space
Compare two strings after applying backspace characters in O(1) space by scanning from right with skip counters.
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Crawler Log Folder — Stack Depth Counter
Track the minimum number of operations to return to the main folder by simulating directory navigation with a depth counter.
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Number of Recent Calls — Sliding Window Queue
Count ping requests within the last 3000ms using a queue that slides expired entries off the front.
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Make the String Great — Stack Same-Letter Case Check
Remove adjacent pairs of same letters with different cases using a stack to build the result character by character.
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Daily Temperatures — Monotonic Decreasing Stack
Find how many days until a warmer temperature using a monotonic decreasing stack of unresolved day indices.
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Next Greater Element I — Monotonic Stack with HashMap
Find the next greater element for each query in O(n+m) using a monotonic stack on nums2 and a lookup map.
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Next Greater Element II — Circular Array Monotonic Stack
Find the next greater element in a circular array by iterating twice (0 to 2n) and using a monotonic stack.
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Online Stock Span — Monotonic Stack with Accumulated Span
Calculate the stock price span (consecutive days <= today) using a monotonic stack that accumulates spans.
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Remove K Digits — Monotonic Increasing Stack
Remove k digits to form the smallest number by maintaining a monotonic increasing stack and greedy removal.
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Decode String — Stack for Counts and Strings
Decode a string with nested encodings like 3[a2[bc]] using two stacks: one for counts and one for partial strings.
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Evaluate Reverse Polish Notation — Operand Stack
Evaluate an RPN expression by pushing numbers and applying operators to the top two stack elements.
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Sliding Window Maximum — Monotonic Deque
Find the maximum in each sliding window of size k using a monotonic decreasing deque storing indices.
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Asteroid Collision — Stack Simulation
Simulate asteroid collisions using a stack where right-moving asteroids wait and left-moving ones destroy smaller ones.
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Sum of Subarray Minimums — Monotonic Stack with Contribution
Sum the minimums of all subarrays by computing each element's contribution as the minimum using previous and next smaller element spans.
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Score of Parentheses — Stack Depth Doubling
Calculate the score of balanced parentheses where () = 1 and AB = A+B and (A) = 2*A using a stack depth trick.
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Task Scheduler — Greedy with Most Frequent First
Find the minimum intervals to execute all tasks with cooldown n by greedily scheduling the most frequent task first.
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Rotting Oranges — Multi-Source BFS
Find minutes until all oranges rot using multi-source BFS starting from all initially rotten oranges simultaneously.
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Number of Islands — BFS or DFS Flood Fill
Count islands in a 2D grid using BFS or DFS to flood-fill connected land cells, marking them visited.
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Course Schedule — Topological Sort BFS (Kahns Algorithm)
Detect if all courses can be finished (no cycle) using Kahn's algorithm: BFS on in-degree zero nodes.
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Course Schedule II — Topological Order via BFS
Return a valid course order using Kahn's BFS topological sort, or empty array if a cycle exists.
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Word Ladder — BFS Shortest Transformation Path
Find the shortest word transformation sequence using BFS where each step changes exactly one character.
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Basic Calculator II — Stack with Operator Precedence
Evaluate a string expression with +, -, *, / using a stack that handles operator precedence without parentheses.
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132 Pattern — Monotonic Stack from Right
Detect a 132 pattern (i < j < k, nums[i] < nums[k] < nums[j]) using a monotonic stack scanning right to left.
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Number of Visible People in Queue — Monotonic Stack
Count visible people in a queue for each person using a monotonic decreasing stack scanning right to left.
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Design Circular Queue — Array with Head and Tail Pointers
Implement a circular queue using a fixed-size array with head and tail pointers and a size counter.
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Pacific Atlantic Water Flow — Reverse BFS from Both Oceans
Find cells that can flow to both oceans by doing reverse BFS from each ocean's borders and finding the intersection.
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Jump Game VI — DP + Monotonic Deque
Maximize score jumping through an array with window k by combining DP with a monotonic decreasing deque.
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Shortest Subarray with Sum at Least K — Deque + Prefix Sum
Find the shortest subarray with sum >= k using a monotonic deque on prefix sums for O(n) time.
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Walls and Gates — Multi-Source BFS from All Gates
Fill each empty room with its distance to nearest gate using simultaneous multi-source BFS from all gates.
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Flatten Nested List Iterator — Stack-Based Lazy Expansion
Implement an iterator for a nested list using a stack that lazily expands nested lists as elements are consumed.
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Remove All Adjacent Duplicates in String II — Stack with Counts
Remove k adjacent identical characters repeatedly using a stack that tracks (character, count) pairs.
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Design Hit Counter — Queue with Timestamp Expiry
Design a hit counter that returns hits in the past 5 minutes using a queue to expire old timestamps.
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Largest Rectangle in Histogram — Monotonic Increasing Stack
Find the largest rectangle in a histogram by computing left and right boundaries using a monotonic increasing stack.
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Maximal Rectangle — Histogram Stack on Each Row
Find the maximal rectangle in a binary matrix by building histogram heights row by row and applying largest rectangle.
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Basic Calculator I — Stack for Sign and Parentheses
Evaluate a basic expression with +, -, and parentheses using a stack to save and restore sign context.
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Maximum Frequency Stack — Bucket-per-Frequency Design
Design a frequency stack with O(1) push and pop-max-frequency using a map of frequency to stack of elements.
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Find Median from Data Stream — Two Heaps
Find the median dynamically as numbers are added using two heaps: a max-heap for the lower half and min-heap for upper.
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Serialize and Deserialize Binary Tree — BFS Level Order
Serialize a binary tree to a string using BFS level-order and deserialize back using a queue for reconstruction.
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Trapping Rain Water — Monotonic Stack Horizontal Layer
Calculate trapped rainwater volume using a monotonic stack that computes water in horizontal layers as bars are processed.
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Stacks & Queues Master Recap — All Patterns & Templates
Complete cheatsheet for Stacks & Queues: all 7 patterns, template code, Big O reference, and MAANG interview priority guide.
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async11 min read
AI Background Processing — Async LLM Jobs, Queues, and Webhook Callbacks
Build scalable AI background processing with BullMQ, idempotent job tracking, exponential backoff, progress streaming, and webhook callbacks for reliable async workflows.
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Design Hit Counter — Count Requests in Last 300 Seconds
Design a hit counter that counts hits in the last 5 minutes using a deque-based sliding window or circular buffer with O(1) amortised operations.
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Design Phone Directory — Available Number Pool
Design a phone directory managing available and allocated numbers with O(1) get, check, and release using a queue of free numbers and a boolean availability array.
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Design Circular Queue — Ring Buffer Implementation
Implement a circular queue (ring buffer) with fixed capacity supporting enQueue, deQueue, Front, Rear, isEmpty, and isFull in O(1) time using head and tail pointers.
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