Mastering Two Pointers & Sliding Window Techniques in Coding Interviews

if you’re prepping for coding interviews, you’ve likely come across problems like:

“Find two numbers in an array that add up to a target.”

“Find the longest substring without repeating characters.”

“Find the maximum sum of a subarray of size k.”

Guess what? Most of these can be solved using two powerful techniques: Two Pointers and Sliding Window.

Let’s break them down and see how they can save you from brute force nightmares

What is the Two Pointers Technique?

This technique involves using two indices (pointers) to iterate through a data structure. It’s especially useful for:

Sorted arrays

Linked lists

Comparing elements from both ends

Example 1: Two Sum (Sorted Array)

def two_sum_sorted(nums, target):
    left, right = 0, len(nums) - 1

    while left < right:
        curr_sum = nums[left] + nums[right]
        if curr_sum == target:
            return [left, right]
        elif curr_sum < target:
            left += 1
        else:
            right -= 1
    return []

Time Complexity: O(n)

What is the Sliding Window Technique?

Used for problems involving subarrays or substrings — you “slide” a window (range) over the data to keep track of a condition (e.g., sum, length, uniqueness).

Example 2: Maximum Sum of Subarray of Size k

def max_sum_subarray(nums, k):
    max_sum = window_sum = sum(nums[:k])
    for i in range(k, len(nums)):
        window_sum += nums[i] - nums[i - k]
        max_sum = max(max_sum, window_sum)
    return max_sum

Time Complexity: O(n)

Example 3: Longest Substring Without Repeating Characters

def length_of_longest_substring(s):
    seen = set()
    left = max_len = 0

    for right in range(len(s)):
        while s[right] in seen:
            seen.remove(s[left])
            left += 1
        seen.add(s[right])
        max_len = max(max_len, right - left + 1)

    return max_len

This is both Two Pointers + Sliding Window magic

When to Use What?

Final Tips

Sliding window is like an adjustable “view” into your array or string.

Two pointers are perfect when the data is sorted or needs comparing from both ends.

Practice makes patterns — soon you’ll spot these problems in seconds.

Practice Problems

Leetcode 3: Longest Substring Without Repeating Characters

Leetcode 209: Minimum Size Subarray Sum

Leetcode 167: Two Sum II – Input Array is Sorted

Hope this helps you in your coding journey!
Feel free to drop your thoughts or questions in the comments below.