Two Pointers vs Sliding Window
Quick Answer
Use Two Pointers when the data is sorted and you need to find pairs, compare elements at opposite ends, or partition the array. Use Sliding Window when you need to find a contiguous subarray or substring that satisfies some constraint (max sum, unique characters, etc.).
Side-by-Side Comparison
| Aspect | Two Pointers | Sliding Window |
|---|---|---|
| Data type | Arrays, strings, linked lists | Arrays, strings |
| Sort required? | Often yes (for pair-finding) | No |
| Window size | Not applicable (pointers converge or diverge) | Fixed or variable contiguous range |
| Pointer movement | Move based on comparison (too big/small) | Expand right, shrink left based on constraint |
| Time complexity | O(n) or O(n log n) with sort | O(n) |
| Space complexity | O(1) | O(1) to O(k) depending on tracking |
| Key signal | "Sorted array", "find a pair", "compare two ends" | "Contiguous subarray", "substring with constraint" |
When to Use Two Pointers
- -The array is sorted (or you can sort it) and you need to find pairs that sum to a target.
- -You need to compare elements from opposite ends of the array (palindrome check, container problems).
- -You need to partition elements in-place (Dutch National Flag, move zeroes).
- -You are working with linked lists (fast/slow pointer is a variant).
When to Use Sliding Window
- -You need the max/min sum or length of a contiguous subarray satisfying some condition.
- -The problem asks for the longest/shortest substring with specific character constraints.
- -You have a fixed window size k and need to compute something for each window position.
- -The answer involves a contiguous range of elements, not arbitrary pairs.
Key Differences
Pointer relationship
In Two Pointers, pointers typically start at opposite ends and move toward each other (or one fast, one slow). In Sliding Window, both pointers move in the same direction — the right pointer expands the window and the left pointer contracts it.
What they track
Two Pointers compares individual elements at the pointer positions. Sliding Window maintains a running aggregate (sum, count, frequency map) of everything between the two pointers.
Sortedness
Two Pointers usually requires sorted input to guide pointer movement. Sliding Window works on unsorted data because it tracks the window's contents via a running state.
How to Decide
- Is the input sorted or should be sorted? Lean toward Two Pointers.
- Does the answer involve a contiguous subarray/substring? Lean toward Sliding Window.
- Are you comparing two specific elements? Two Pointers.
- Are you tracking an aggregate (sum, count, unique chars) over a range? Sliding Window.
- Does the problem mention "pair", "triplet", or "target sum"? Two Pointers (after sorting).
- Does the problem mention "maximum length", "minimum window", or "at most k"? Sliding Window.
Example Problems
Two Pointers
- 1.3Sum — Sort the array, fix one element, and use two pointers on the remaining range to find triplets summing to zero.
- 2.Container With Most Water — Two pointers at both ends, move the shorter side inward to maximize area.
- 3.Valid Palindrome — Compare characters from both ends of the string, moving inward.
Sliding Window
- 1.Maximum Sum Subarray of Size K — Fixed-size window: slide across the array, add the new element, remove the old one.
- 2.Longest Substring Without Repeating Characters — Variable window: expand right, shrink left when a duplicate is found.
- 3.Minimum Window Substring — Variable window: expand to include all target chars, shrink to find the smallest valid window.