Two Pointers Questions at Turing: What to Expect

Two Pointers questions appear in 10% of Turing’s technical interviews (4 out of 40 problems). While not the most frequent pattern, it’s a consistent and high-signal topic. Turing uses these problems to assess a candidate’s ability to optimize brute-force solutions, handle edge cases in array/string manipulation, and write clean, efficient code under time pressure. Mastering this pattern demonstrates you can think logically about spatial relationships and complexity—a skill that translates directly to real-world performance optimization tasks.

What to Expect — Types of Problems

At Turing, Two Pointers questions typically fall into two categories:

1. Opposite-direction pointers: Used on sorted arrays or strings to find pairs, triples, or validate conditions (e.g., two-sum in a sorted array, palindrome checking).
2. Fast-slow pointers: Applied to linked lists or sequences to detect cycles, find midpoints, or solve problems like removing duplicates from a sorted array in-place. You will likely encounter problems involving arrays or strings, often with a requirement to solve in O(n) time and O(1) extra space. Common themes include deduplication, partitioning, and searching for subarrays that meet a certain sum or condition.

How to Prepare — Study Tips with One Code Example

Focus on understanding the conditions that make a two-pointer approach applicable: sorted data, in-place operations, or cycle detection. Start by solving the brute-force solution, then identify how two pointers can reduce complexity. Always test with edge cases: empty input, single element, all duplicates, or already sorted/partitioned data.

A key pattern is the opposite-direction pointer for finding a pair with a target sum in a sorted array. Here’s the implementation:

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 proficiency incrementally:

1. Basics: Remove duplicates from sorted array, valid palindrome.
2. Pair searching: Two-sum in sorted array, three-sum.
3. In-place operations: Move zeroes, sort colors (Dutch national flag).
4. Linked list: Detect cycle, find middle node.
5. Advanced: Container with most water, trapping rain water (variants using two pointers). This order ensures you grasp the fundamental movement logic before tackling problems that combine two pointers with other techniques like sliding window or greedy selection.

Practice Two Pointers at Turing