Data Structures Practice Test
Ten questions on stacks, queues, trees and time complexity for computer science exams and placement preparation.
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Question 1 of 10
Which data structure follows Last In, First Out (LIFO)?
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Answer: Stack. A stack removes the most recently added item first, like a pile of plates.
Question 2 of 10
Which data structure follows First In, First Out (FIFO)?
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Answer: Queue. A queue removes items in the order they were added, like a line at a counter.
Question 3 of 10
What is the worst-case time complexity of binary search on a sorted array?
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Answer: O(log n). Each step halves the search range, so it takes about log₂ n steps.
Question 4 of 10
What is the time complexity of reading an array element by its index?
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Answer: O(1). The element's address can be calculated directly from its index, so access takes constant time.
Question 5 of 10
Which data structure is normally used for breadth-first search (BFS) on a graph?
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Answer: Queue. BFS visits nodes level by level, and a queue keeps them in that order. Depth-first search uses a stack.
Question 6 of 10
Which traversal of a binary search tree gives its values in sorted order?
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Answer: Inorder. Inorder visits left, node, right. In a binary search tree this gives ascending order.
Question 7 of 10
What is the worst-case time complexity of merge sort?
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Answer: O(n log n). Merge sort always splits the list in half and merges in linear time, giving O(n log n).
Question 8 of 10
What is the average time complexity of looking up a key in a hash table?
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Answer: O(1). With a good hash function, lookups take constant time on average. The worst case can reach O(n).
Question 9 of 10
In a min-heap, where is the smallest element?
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Answer: At the root. Every parent is smaller than or equal to its children, so the minimum is always at the root.
Question 10 of 10
How long does it take to insert a node at the head of a singly linked list?
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Answer: O(1). You only point the new node at the old head and update the head, so no traversal is needed.
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