Two Pointers vs Sliding Window: When to Use Each

Two Pointers and Sliding Window are among the most frequently asked patterns in coding interviews. Both use two indices to scan through an array or string, so beginners often confuse them. But they solve fundamentally different types of problems. This guide breaks down when to use each, shows side-by-side code, and gives you a decision framework you can apply in interviews.

The Core Difference

Two Pointers typically uses pointers at opposite ends of a sorted array, moving them toward each other based on a condition. It answers questions about pairs or comparisons from both ends.

Sliding Window uses two pointers that move in the same direction to maintain a dynamic range (window). It answers questions about contiguous subarrays or substrings.

Side-by-Side Comparison

Decision Framework

Code Example: Two Pointers

Problem: Given a sorted array, find two numbers that add up to a target.

function twoSumSorted(nums: number[], target: number): number[] {
  let left = 0;
  let right = nums.length - 1;

  while (left < right) {
    const sum = nums[left] + nums[right];
    if (sum === target) {
      return [left, right];
    } else if (sum < target) {
      left++;   // need a larger sum
    } else {
      right--;  // need a smaller sum
    }
  }
  return [];
}

The pointers start at opposite ends. Because the array is sorted, moving left right increases the sum, and moving right left decreases it. This gives us O(n) instead of the brute-force O(n²).

Code Example: Sliding Window

Problem: Find the length of the longest substring without repeating characters.

function lengthOfLongestSubstring(s: string): number {
  const seen = new Map<string, number>();
  let left = 0;
  let maxLen = 0;

  for (let right = 0; right < s.length; right++) {
    if (seen.has(s[right]) && seen.get(s[right])! >= left) {
      left = seen.get(s[right])! + 1;  // shrink window
    }
    seen.set(s[right], right);
    maxLen = Math.max(maxLen, right - left + 1);
  }
  return maxLen;
}

Both pointers move left to right. The right pointer expands the window, and the left pointer shrinks it when a duplicate is found. The hash map tracks the last position of each character.

Example Problems for Each Pattern

Common Mistakes

Mistake 1: Using Sliding Window on unsorted pair problems. If the problem asks "find two numbers that sum to X" in a sorted array, Two Pointers is faster and simpler than maintaining a window.

Mistake 2: Using Two Pointers for substring problems. If you are looking for a "longest substring with at most K distinct characters," that is a Sliding Window problem. The pointers move in the same direction, not toward each other.

Mistake 3: Forgetting the sorted requirement for Two Pointers. The converging Two Pointers technique only works when the array is sorted (or has a monotonic property). If the input is unsorted and you need a pair with a target sum, use the Hash Map / Two Sum Pattern instead.

Quick Cheat Sheet

Want to go deeper on Sliding Window? Read our guide on How to Identify Sliding Window Problems. Or explore all patterns on the Cheat Sheet.