Sliding Window vs Two Pointers: How to immediately recognize the difference in interview questions?

Question

Both techniques use two pointers (left and right) on arrays or strings. What is the fundamental test to decide whether a problem is Sliding Window or Two Pointers?

Ankit Yadav · Accepted Answer

Here is the exact rule of thumb that eliminates confusion:

### 1. Sliding Window (Subarrays & Substrings)
- **Look for keywords:** *"contiguous subarray"*, *"substring"*, *"window of size K"*, *"maximum/minimum sum of length K"*.
- **How it moves:** Both pointers move in the **same direction** (usually left-to-right). The right pointer expands the window to satisfy a condition; the left pointer contracts it when the window becomes invalid.
- **Classic Examples:** Minimum Window Substring, Longest Substring Without Repeating Characters, Subarray Product Less Than K.

### 2. Two Pointers (Opposite Ends or Sorted Arrays)
- **Look for keywords:** *"sorted array"*, *"pair with target sum"*, *"reverse string"*, *"palindrome verification"*, *"partition array"*.
- **How it moves:** Pointers usually start at **opposite ends** (`left = 0`, `right = n - 1`) and move toward each other, or move at different speeds (Fast & Slow pointer for cycle detection).
- **Classic Examples:** 2Sum II (Sorted), 3Sum, Container With Most Water, Trapping Rain Water.

**Summary:** If it asks for a **contiguous segment**, think **Sliding Window**. If it asks to **find pairs or converge from ends on sorted data**, think **Two Pointers**.

Karan Patel · Answer

This makes it crystal clear. 'Contiguous segment = sliding window'.

Sliding Window vs Two Pointers: How to immediately recognize the difference in interview questions?

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