Two pointers: a practical technique for code challenges

Tech interviews are tougher than ever, and landing a job means showing up prepared. That's exactly why we're breaking down essential, interview-focused concepts that consistently appear in technical screenings to help the Vets Who Code community build familiarity and confidence.
Today, we're diving into the Two Pointers technique – a powerful, practical approach that can optimize your code and impress interviewers.

What Exactly Is the Two Pointers Technique?

At its core, the Two Pointers technique is a problem-solving strategy where you use two variables (our "pointers") to efficiently traverse a list or string. Instead of juggling multiple indices or iterating in a complex way, these two pointers streamline your scan.
Here's how they typically work:

Opposite Ends: The pointers start at the very beginning and very end of your sequence, moving inward towards each other.
Moving Together: Both pointers can start at the same point (often the beginning) and advance forward in parallel.

Why use this? Simple: it often replaces clunky nested loops, leading to significant performance gains and minimal memory usage. Most Two Pointer solutions boast an impressive linear time complexity (O(n)) and use constant space (O(1)). That's exactly what interviewers love to see!

Pattern 1: Pointers From Opposite Ends

This pattern shines when you need to compare or interact with values from both extremes of a sequence. Think problems involving symmetry, or when you need to find a pair that meets certain criteria.

Example 1: Palindrome Check

A classic use case! You want to know if a string reads the same backward and forward.

Python:

def is_palindrome(text):
    start_index = 0
    end_index = len(text) - 1

    while start_index < end_index:
        if text[start_index] != text[end_index]:
            return False
        start_index += 1
        end_index -= 1

    return True

JavaScript:

function isPalindrome(text) {
    let startIndex = 0;
    let endIndex = text.length - 1;

    while (startIndex < endIndex) {
        if (text[startIndex] !== text[endIndex]) {
            return false;
        }
        startIndex += 1;
        endIndex -= 1;
    }

    return true;
}

Example 2: Find a Pair That Sums to a Target

Given a sorted array, can you find if any two numbers within it add up to a specific target?

Python:

def has_pair_with_sum(numbers, target_sum):
    left_pointer = 0
    right_pointer = len(numbers) - 1

    while left_pointer < right_pointer:
        current_sum = numbers[left_pointer] + numbers[right_pointer]

        if current_sum == target_sum:
            return True

        if current_sum < target_sum:
            left_pointer += 1
        else:
            right_pointer -= 1

    return False

JavaScript:

function hasPairWithSum(numbers, targetSum) {
    let leftPointer = 0;
    let rightPointer = numbers.length - 1;

    while (leftPointer < rightPointer) {
        const currentSum = numbers[leftPointer] + numbers[rightPointer];

        if (currentSum === targetSum) {
            return true;
        }

        if (currentSum < targetSum) {
            leftPointer += 1;
        } else {
            rightPointer -= 1;
        }
    }

    return false;
}

Pattern 2: Pointers Moving Together

This pattern is your go-to when you're comparing two separate sequences, or when you need two "roles" scanning a single sequence simultaneously. It's often seen in merging, matching, or subsequence problems.

Example 3: Merge Two Sorted Arrays

Combine two sorted arrays into one perfectly sorted array.

Python:

def merge_sorted_arrays(first_array, second_array):
    first_index = 0
    second_index = 0
    merged_array = []

    while first_index < len(first_array) and second_index < len(second_array):
        if first_array[first_index] < second_array[second_index]:
            merged_array.append(first_array[first_index])
            first_index += 1
        else:
            merged_array.append(second_array[second_index])
            second_index += 1

    while first_index < len(first_array):
        merged_array.append(first_array[first_index])
        first_index += 1

    while second_index < len(second_array):
        merged_array.append(second_array[second_index])
        second_index += 1

    return merged_array

JavaScript:

function mergeSortedArrays(firstArray, secondArray) {
    let firstIndex = 0;
    let secondIndex = 0;
    const mergedArray = [];

    while (firstIndex < firstArray.length && secondIndex < secondArray.length) {
        if (firstArray[firstIndex] < secondArray[secondIndex]) {
            mergedArray.push(firstArray[firstIndex]);
            firstIndex += 1;
        } else {
            mergedArray.push(secondArray[secondIndex]);
            secondIndex += 1;
        }
    }

    while (firstIndex < firstArray.length) {
        mergedArray.push(firstArray[firstIndex]);
        firstIndex += 1;
    }

    while (secondIndex < secondArray.length) {
        mergedArray.push(secondArray[secondIndex]);
        secondIndex += 1;
    }

    return mergedArray;
}

Example 4: Subsequence Check

Want to know if all the characters in one string appear in another, in the same order?

Python:

def is_subsequence(candidate, source):
    candidate_index = 0
    source_index = 0

    while candidate_index < len(candidate) and source_index < len(source):
        if candidate[candidate_index] == source[source_index]:
            candidate_index += 1
        source_index += 1

    return candidate_index == len(candidate)

JavaScript:

function isSubsequence(candidate, source) {
    let candidateIndex = 0;
    let sourceIndex = 0;

    while (candidateIndex < candidate.length && sourceIndex < source.length) {
        if (candidate[candidateIndex] === source[sourceIndex]) {
            candidateIndex += 1;
        }
        sourceIndex += 1;
    }

    return candidateIndex === candidate.length;
}

How to Spot a Two Pointers Problem

Look for these indicators:

The input is a sorted array
You're asked to find a pair with a specific sum or difference
The problem involves a palindrome or symmetry
You're merging or comparing two sequences
The prompt mentions a subsequence check
You're scanning two strings or arrays at the same time
The goal is O(n) time and O(1) space

Ask yourself:

Is the data sorted?
Am I comparing ends?
Am I scanning in sync?
Can I avoid a nested loop?

Why This Matters for Your Tech Career

The Two Pointers technique isn’t just a clever trick – it demonstrates:

Efficient algorithm design
Time and space complexity awareness
Practical problem-solving skills

By mastering this, you’re preparing yourself for coding interviews that test your depth and clarity.

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