aloalgoThe Two Pointer Technique Explained
Two pointers is one of the most versatile techniques in coding interviews. The idea is simple: use two pointers to traverse a data structure, avoiding the need for nested loops. This often reduces O(n²) solutions to O(n).1. The Three Patterns
Almost every two pointer problem falls into one of these categories:| Pattern | Pointers Start | Movement | Classic Problems |
|---|---|---|---|
| Opposite Ends | Start and end | Move toward each other | Two Sum (sorted), Container With Water |
| Same Direction | Both at start | One fast, one slow | Remove duplicates, Merge arrays |
| Fast & Slow | Both at start | Different speeds | Cycle detection, Find middle |
2. Opposite Ends Pattern
Start with pointers at both ends of a sorted array. Move them toward each other based on some condition.Template
Two Sum (Sorted Array)
Find two numbers that add up to target in a sorted array.Container With Most Water
Find two lines that form a container holding the most water.Valid Palindrome
3Sum
Find all triplets that sum to zero. Fix one element, then use two pointers on the rest.3. Same Direction Pattern
Both pointers start at the beginning. One pointer (fast) explores ahead, while the other (slow) marks a position.Template
Remove Duplicates from Sorted Array
Move Zeroes to End
Remove Element
Merge Two Sorted Arrays
4. Fast & Slow Pointer (Floyd's)
One pointer moves twice as fast as the other. Used for cycle detection and finding midpoints in linked lists.Detect Cycle in Linked List
Find Cycle Start
Find Middle of Linked List
Palindrome Linked List
Use fast/slow to find middle, reverse second half, compare.5. When to Use Two Pointers
| Signal | Pattern |
|---|---|
| Sorted array + find pair | Opposite ends |
| Palindrome check | Opposite ends |
| In-place array modification | Same direction (slow/fast) |
| Merging sorted arrays | Same direction |
| Linked list cycle | Fast & slow |
| Find middle of list | Fast & slow |
| Reduce O(n²) to O(n) | Usually opposite ends |
6. Two Pointers vs Sliding Window
They're related but different:| Two Pointers | Sliding Window | |
|---|---|---|
| Goal | Find pair/triplet satisfying condition | Find subarray satisfying condition |
| Typical movement | Toward each other or same direction | Always same direction (right expands) |
| Data usually | Sorted | Not necessarily sorted |
| Example | Two Sum in sorted array | Longest substring without repeats |
7. Complexity
| Pattern | Time | Space |
|---|---|---|
| Opposite ends | O(n) | O(1) |
| Same direction | O(n) | O(1) |
| Fast & slow | O(n) | O(1) |
| 3Sum (with sorting) | O(n²) | O(1) |
8. Practice Problems
Opposite EndsFast & Slow (Linked List)MergingFinal Thoughts
Two pointers is really about eliminating redundant comparisons. Instead of checking every pair (O(n²)), you use the structure of the problem—usually sorted order—to skip comparisons you know won't work. The key is recognizing which direction to move each pointer based on your current state.Was this helpful?
Company
Practice
© 2026 aloalgo. All rights reserved.