Two Pointers Questions at Hashedin: What to Expect

Two Pointers questions appear in roughly 12.5% of Hashedin’s technical interview problems (4 out of 32). This frequency means you are highly likely to encounter at least one such problem. Mastering this pattern is non-negotiable for efficient problem-solving, as it transforms brute-force O(n²) solutions into optimal O(n) ones, directly demonstrating your ability to optimize time and space complexity—a key trait Hashedin assesses.

What to Expect — Types of Problems

Hashedin’s Two Pointers problems typically fall into three categories. First, opposite-direction pointers are used for problems like finding a pair with a target sum in a sorted array or checking for a palindrome. Second, same-direction (fast-slow) pointers appear in linked list cycles or removing duplicates from a sorted array in-place. Third, sliding window variants might be framed as finding a subarray meeting a condition. Expect the problems to be applied to arrays, strings, or linked lists. The difficulty often lies in correctly moving the pointers to maintain invariants without off-by-one errors.

How to Prepare — Study Tips with One Code Example

Focus on understanding the core mechanic: using two indices to traverse a data structure, often reducing the problem space with each move. Start by identifying when the pattern applies: sorted data, pair searching, or in-place manipulation. Practice drawing the pointer movements on a whiteboard. For a key pattern like finding a target sum pair in a sorted array, internalize this solution:

def two_sum_sorted(numbers, target):
    left, right = 0, len(numbers) - 1
    while left < right:
        current_sum = numbers[left] + numbers[right]
        if current_sum == target:
            return [left, right]
        elif current_sum < target:
            left += 1
        else:
            right -= 1
    return [-1, -1]

Recommended Practice Order

Build competency progressively. Start with fundamental opposite-direction problems (Two Sum II, Valid Palindrome). Move to same-direction problems (Remove Duplicates from Sorted Array, Linked List Cycle). Then tackle sliding window problems (Minimum Size Subarray Sum). Finally, solve Hashedin’s specific tagged problems to familiarize yourself with their presentation and difficulty level. Always analyze time/space complexity aloud during practice to build interview habits.

Practice Two Pointers at Hashedin