Two Pointers

Master the two-pointer technique for arrays, strings, and linked lists. Learn opposite direction, fast/slow, and in-place manipulation patterns.

The Two Pointers technique transforms $O (N^{2})$ brute force solutions into efficient $O (N)$ algorithms by using two indices to traverse data structures intelligently. Master opposite-direction pointers for sorted arrays, fast/slow pointers for linked lists, and in-place manipulation patterns. Learn when to use each variant and how to recognize the pattern instantly.

Featured Guide

Complete Guide

The Complete Two Pointers Guide: Master All Patterns, Templates, and Techniques

The ultimate comprehensive guide to two pointers. Learn all variants, when to use each pattern, complete templates in multiple languages, and a systematic approach to solve any two pointers problem.

25 min read

Interactive Tool

Roadmap

Two Pointers Visualizer

Master the two pointers technique through interactive visualization. Step through opposite-direction, read/write, and fast/slow algorithms in real-time. See pointer movements, variable updates, and pattern mechanics.

Two Pointers Learning Roadmap: From Beginner to Interview-Ready in 8 Weeks

A structured 8-week learning path to master all two pointers patterns. Progress from basic concepts to advanced techniques with curated problems, templates, and milestones.

Tutorials

Why Your Two Pointers Solution Fails on Unsorted Arrays (And What to Use Instead)

Applying two pointers to unsorted arrays is a common interview mistake that leads to wrong answers. Learn why sorting is required, when to use hash maps instead, and how to recognize the difference in 30 seconds.

Two PointersHash MapLeetCode Pattern

9 min

The Off-by-One Trap in Two Pointers: How to Get Pointer Bounds Right Every Time

Off-by-one errors in two pointers solutions cause subtle bugs that pass most test cases but fail edge cases. Learn the systematic approach to correct pointer initialization, loop bounds, and window size calculation.

Two PointersDebuggingCommon Mistakes

10 min

How to Debug Two Pointers Solutions: Visualization and Common Patterns

Stuck on a two pointers problem with no idea what's wrong? Learn practical debugging techniques, visualization strategies, and systematic approaches to trace pointer movement and identify bugs fast.

Two PointersDebuggingVisualization

11 min

Fast and Slow Pointers: Why the Slow Pointer Always Catches the Fast Pointer in a Cycle

Floyd's cycle detection algorithm seems like magic—but there's rigorous math behind it. Learn the intuitive explanation and mathematical proof of why the tortoise always catches the hare.

Two PointersFloyd's AlgorithmCycle Detection

12 min

3Sum Duplicate Handling: The Edge Case That Breaks Most Solutions

Your 3Sum solution works on simple test cases but fails when duplicates appear. Learn the exact logic for skipping duplicates in 3Sum, why it's necessary, and how to implement it correctly.

Two Pointers3SumDuplicates

11 min

When to Use Hash Map Instead of Two Pointers (Decision Framework)

Confused whether to use a hash map or two pointers for pair-finding problems? Learn the exact decision criteria, trade-offs, and when each approach wins.

Two PointersHash MapDecision Framework

10 min

Two Pointers on Linked Lists: Common Pitfalls and Null Pointer Errors

Linked list two pointers problems crash with null pointer exceptions more than any other pattern. Learn safe pointer advancement, null checks, and how to avoid the most common mistakes.

Two PointersLinked ListNull Pointer

10 min

Container With Most Water: Why Greedy Two Pointers Works (Proof)

The greedy approach to Container With Most Water seems counterintuitive—why move the shorter line? Learn the intuitive explanation and rigorous proof of why this algorithm is correct.

Two PointersGreedy AlgorithmProof

11 min

Remove Duplicates from Sorted Array: The Write Pointer Pattern Explained

The read/write pointer pattern for in-place array manipulation is confusing at first. Learn the intuition, step-by-step walkthrough, and why this pattern works for removing duplicates.

Two PointersIn-PlaceArray Manipulation

10 min

Move Zeroes: Why Order Matters in Two Pointers In-Place Swaps

Your Move Zeroes solution works but fails test cases because it doesn't preserve relative order. Learn the difference between naive swapping and stable partitioning.

Two PointersStable PartitionIn-Place

10 min

Two Pointers Pattern Recognition: The 5-Second Checklist for Interviews

Learn to identify two pointers problems instantly with a systematic checklist. Master the keywords, problem types, and decision framework that separate fast solvers from slow ones.

Two PointersPattern RecognitionInterview Strategy

11 min

Valid Palindrome Edge Cases: Empty Strings, Single Characters, and Non-Alphanumeric

Your Valid Palindrome solution works on simple inputs but fails edge cases. Learn comprehensive edge case handling for empty strings, single characters, spaces, and special characters.

Two PointersPalindromeEdge Cases

10 min

Opposite Direction Two Pointers: Template, Examples, and When to Use It

Master the converging pointers pattern with production-ready templates in Python, JavaScript, and Java. Learn when to use opposite direction two pointers, common patterns, and how to avoid mistakes.

Two PointersOpposite DirectionSorted Arrays

15 min

Fast and Slow Pointers: The Complete Guide for Linked Lists and Arrays

Master the tortoise and hare algorithm with comprehensive templates, mathematical proofs, and real-world applications. Learn cycle detection, finding middle elements, and advanced techniques.

Two PointersFast SlowLinked List

16 min

Two Pointers vs Sliding Window: When to Use Which Pattern

Stop confusing two pointers with sliding window. Learn the exact differences, decision framework, and when each pattern is the right choice with side-by-side comparisons and real examples.

Two PointersSliding WindowPattern Recognition

14 min

Two Pointers on Sorted Arrays: Why Sorting Unlocks O(n) Solutions

Discover why sorting transforms impossible problems into trivial ones. Learn when to sort first, the mathematical proof of correctness, and how to handle the indices vs values trade-off.

Two PointersSorted ArraysSorting

16 min

Two Pointers for In-Place Array Manipulation: Remove, Partition, and Swap

Master the read/write pointer pattern for O(1) space array manipulation. Learn when to use in-place techniques, how to preserve order, and the patterns that appear in dozens of problems.

Two PointersIn-PlaceArray Manipulation

15 min

Advanced Two Pointers: 3Sum, 4Sum, and Multi-Pointer Techniques

Master advanced multi-pointer patterns that extend beyond basic two pointers. Learn 3Sum, 4Sum, k-Sum generalization, and how to handle duplicates at scale.

Two Pointers3Sum4Sum

16 min