Hash Map

Master hash map patterns for O(1) lookup problems. Learn frequency counting, two sum, grouping, index mapping, and prefix sum combinations.

The Hash Map pattern transforms $O (N^{2})$ brute force solutions into $O (N)$ algorithms by trading space for time. Master the 6 core patterns: frequency counter, two sum/complement search, grouping/categorization, index mapping, prefix sum + hash map, and seen tracker. Learn when to use hash maps vs sets, arrays, trees, and two pointers.

Featured Guide

Complete Guide

Hash Map Pattern: Complete Guide With Templates (Python/Java/JS)

Master the hash map pattern beyond basic dictionaries. Learn all 6 core patterns, recognition strategies, multi-language templates, and the decision framework that turns O(n²) brute force into O(n) elegance.

25 min read

Interactive Tool

Roadmap

Hash Map Visualizer

Master hash map patterns through interactive visualization. Step through Two Sum, Frequency Counting, and Grouping algorithms. Watch how O(1) lookups and category mapping work in real-time.

Hash Map Learning Roadmap: From Beginner to Interview-Ready in 5 Weeks

A structured 5-week learning path to master all hash map patterns. Progress from basic lookups to advanced prefix sum combinations, grouping, and decision frameworks with curated problems and milestones.

Tutorials

5 Common Hash Map Mistakes That Fail Interviews (And How to Fix Them)

Your hash map solution keeps getting 'Wrong Answer.' Learn the 5 most common mistakes that trip up even experienced developers and how to fix them instantly.

Hash MapCommon MistakesDebugging

7 min

How to Choose the Right Hash Map Key (The Decision Framework)

Struggling with 'what should I use as the key?' Learn the systematic 3-step framework for choosing hash map keys that always works.

Hash MapKey SelectionDesign Patterns

7 min

Handling Duplicates in Hash Maps: Store First, Last, or All?

Confused about how to handle duplicate keys? Learn the 4 duplicate strategies and when to use each for different problem types.

Hash MapDuplicatesEdge Cases

7 min

When NOT to Use Hash Maps (5 Cases Where They Fail)

Hash maps are powerful, but not always optimal. Learn the 5 specific scenarios where hash maps fail and what to use instead to ace your interview.

Hash MapData StructuresInterview Strategy

6 min

Hash Map Time Complexity: O(1) Average, O(n) Worst Case Explained

Confused about why hash maps are O(1) if collisions exist? Learn the truth about average vs worst case complexity and how to explain it in interviews.

Hash MapTime ComplexityBig O

6 min

Hash Map vs Two Pointers: When to Use Each for Sum Problems

Stop guessing between hash maps and two pointers. Learn the simple decision rule that works every time for pair and sum problems.

Hash MapTwo PointersDecision Framework

6 min

Group Anagrams: 3 Hash Map Approaches (Sorted, Count, Prime)

Know the basic sorted-string approach? Learn 2 more optimizations and when to use each for Group Anagrams and similar problems.

Hash MapGroup AnagramsOptimization

4 min

Why Your Subarray Sum Equals K Solution Fails (3 Common Bugs)

Getting 'Wrong Answer' on Subarray Sum Equals K? Learn the 3 bugs that trip up 90% of developers and how to fix them instantly.

Hash MapPrefix SumDebugging

7 min

Hash Map Memory Optimization: When and How to Reduce Space

Facing memory limit exceeded? Learn 4 techniques to reduce hash map space usage without sacrificing correctness.

Hash MapMemory OptimizationSpace Complexity

5 min

Using Hash Maps for Cycle Detection (Linked List, Array, Graph)

Know Floyd's algorithm but want to understand the hash map approach? Learn when hash maps are better for cycle detection and the trade-offs.

Hash MapCycle DetectionLinked List

4 min

Combining Hash Maps with Sliding Window (Template & Examples)

Understand both patterns separately but struggle to combine them? Learn the template for hash map + sliding window problems.

Hash MapSliding WindowPattern Combination

5 min

Hash Map vs Hash Set: When to Use Each (Decision Guide)

Stop defaulting to hash maps when sets would suffice. Learn the simple rule for choosing between maps and sets based on what you need to store.

Hash MapHash SetDecision Framework

6 min

Hash Map Frequency Counter: The Complete Pattern Guide

Master the frequency counter pattern beyond basic counting. Learn when to use Counter vs dict, how to find top-k elements efficiently, and the templates that solve 50+ LeetCode problems.

Hash MapFrequency CounterLeetCode Pattern

12 min

Two Sum Hash Map Pattern: Master Complement Search in O(n)

Go beyond memorizing Two Sum. Learn the complement search mental model, when hash maps beat two pointers, how to handle duplicates, and extend to 3Sum/4Sum problems.

Hash MapTwo SumComplement Search

14 min

Hash Map Grouping Pattern: Categorize and Aggregate Data in O(n)

Master the grouping pattern for 'group by' problems. Learn how to choose the perfect grouping key, handle multi-value storage, and solve categorization problems like Group Anagrams efficiently.

Hash MapGroupingCategorization

13 min

Hash Map Index Mapping: Store Positions for O(1) Lookup

Master the index mapping pattern for position tracking. Learn when to store first index, last index, or all indices, and how to optimize space while maintaining O(1) lookup.

Hash MapIndex MappingPosition Tracking

12 min

Prefix Sum + Hash Map: Solve Subarray Problems in O(n)

Learn how combining prefix sums with hash maps unlocks O(n) solutions for subarray problems with negative numbers. Master the pattern that sliding window can't handle.

Hash MapPrefix SumSubarray

14 min

Hash Map vs Array, Set, Tree: When to Use Each

Stop guessing which data structure to use. Learn the systematic decision framework for choosing between hash maps, arrays, sets, trees, and two pointers based on problem requirements.

Hash MapData StructuresComparison

15 min