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Coding Interview Patterns: Arrays, Hash Maps, Two Pointers and Sliding Window — quiz

11 multiple-choice questions covering Two Pointers, Sliding Window (Fixed), Sliding Window (Variable), Prefix Sums, Hash Map / Set Tricks. One answer each; you get a score at the end and the explanation for every question. No account needed.

Updated 2026-11-04. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.

The questions

Answers and explanations are shown after you finish.

1. Two Pointers

   You're using two pointers on a sorted array to find a pair that sums to a target value. The current sum of the pair exceeds the target. What should you do next?

   • Move the left pointer right to increase the sum
   • Restart the search with both pointers at the middle of the array
   • Move the right pointer left to decrease the sum
   • Move both pointers inward simultaneously regardless of the comparison
2. Two Pointers

   Which description correctly matches the two-pointer technique used to remove duplicates from a sorted array in place?

   • A slow pointer marks the last confirmed unique position while a fast pointer scans ahead for new values
   • A single pointer scans while a stack tracks duplicate values
   • Two pointers start at both ends and swap elements while moving inward until they meet
   • Two pointers move based on comparison to a target sum
3. Sliding Window (Fixed)

   To find all starting indices in a string s where an anagram of a shorter pattern p occurs, which approach correctly solves this?

   • Binary search over possible starting indices in s
   • A fixed-size sliding window of length len(p) combined with a character frequency map compared at each position
   • A variable sliding window that grows until a duplicate character appears
   • Sorting every substring of length len(p) and comparing it character by character to sorted p
4. Sliding Window (Variable)

   For finding the length of the smallest contiguous subarray whose sum is at least a target value, which window strategy applies?

   • Start two pointers at both ends of the array and move them inward
   • Shrink from the right while simultaneously expanding from the left
   • Expand the right pointer to grow the sum, then shrink from the left while the sum condition still holds
   • Keep the window at a fixed size equal to the length of the array
5. Sliding Window (Variable)

   In the sliding-window solution for 'longest substring without repeating characters,' what triggers shrinking the window from the left?

   • Encountering a character that's already present in the current window
   • The running sum exceeding a target value
   • The right pointer reaching the end of the string
   • The window reaching a fixed size k
6. Prefix Sums

   Given arr = [1,2,3,4], what is sum(1,3) (sum of elements from index 1 to 3 inclusive) using the prefix sum formula?

   • 7
   • 6
   • 10
   • 9
7. Prefix Sums

   Why is prefix sum combined with a hash map better suited than a sliding window for finding a subarray that sums to exactly k when the array may contain negative numbers?

   • Hash maps are always faster than arrays for any kind of lookup
   • Sliding window relies on the sum changing monotonically as the window grows, which breaks down with negative numbers, while prefix sum with a hash map still works
   • Prefix sums eliminate the need to scan the array at all
   • Sliding window only works when the array is already sorted
8. Hash Map / Set Tricks

   When finding two numbers in an unsorted array that sum to k using a hash map, what do you check at each element?

   • Whether the current element has already been added to the map, and if so, remove it
   • Whether the array has already been sorted in ascending order
   • Whether the current element is less than k
   • Whether the complement (k minus the current element) has already been seen and recorded in the map
9. Fast & Slow Pointers

   After the fast and slow pointers meet inside a linked list cycle, how do you determine the cycle's starting node?

   • Reset one pointer to the head of the list, then advance both pointers one step at a time until they meet again
   • Count the number of nodes between the two pointers at the point they met
   • Reverse the entire linked list and rerun the fast and slow pointer algorithm
   • Continue running the fast pointer alone until it loops back around to the meeting point
10. Kadane's Algorithm

    Why does the maximum product subarray variant of Kadane's algorithm need to track a running minimum in addition to a running maximum?

    • The running minimum is needed to normalize the array before computing products
    • Multiplying two negative numbers can turn the running minimum into the new maximum
    • The minimum is always the final answer whenever the array has an even number of elements
    • Running minimum prevents integer overflow during multiplication
11. Merge Intervals

    Intervals [2,6] and [5,10] are checked using the overlap condition a <= d and c <= b. Do they overlap?

    • No, because 6 is less than 10
    • Yes, because both intervals contain the number 10
    • No, because 2 is less than 5
    • Yes, because 2 <= 10 and 5 <= 6, satisfying the overlap condition