Arrays & Hashing Quiz

8 questions12 minPass: 60%+10 XP on first pass

Hash sets, frequency maps, the Two Sum complement trick, prefix sums and Kadane's algorithm.

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Question 1
Which data structure makes "have I seen this value before?" an O(1) average check?
Sorted arrayHash setLinked listStack
Question 2
In the hash-map solution for Two Sum, what do we look up for each number x?
x itselftarget − xtarget + xx × 2
Question 3
Why do we store x in the map only AFTER checking for target − x in Two Sum?
To keep the map sortedSo an element is never paired with itselfBecause maps cannot store duplicatesTo reduce memory usage

Question 4

What does this code print?

from collections import Counter
c = Counter("banana")
print(c["a"])

Question 5
What is the best time complexity to check whether two strings are anagrams?
O(n²)O(n log n) — sorting is requiredO(n)O(1)
Question 6
prefix = [0, 2, 5, 9, 14] for nums = [2, 3, 4, 5]. What is the sum of nums[1..3] (indices 1 to 3)?
Question 7
What does Kadane's algorithm return for [-2, 1, -3, 4, -1, 2, 1, -5, 4]?
46710
Question 8
For "count subarrays with sum = k" where the array has NEGATIVE numbers, which approach works?
Sliding windowPrefix sums with a hash mapSorting then two pointersBinary search