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Data Structures — Questions and Answers

Data structures decide how efficiently your program stores and retrieves information, and they are the single most-tested topic in software placement rounds. Interviewers expect you to state the time complexity of every basic operation without hesitation, and to justify choosing one structure over another for a specific problem.

8 solved questionsComputer Science EngineeringFree · no signup

Data Structures— Concepts, Formulas & Shortcuts

  • Array: O(1) indexed access, O(n) insert/delete in the middle, fixed size.
  • Linked list: O(1) insert/delete given the node, O(n) search, no random access.
  • Stack is LIFO (push/pop); queue is FIFO (enqueue/dequeue).
  • Binary search on a sorted array is O(log n); linear search is O(n).
  • Hash table gives O(1) average lookup but O(n) worst case when collisions degenerate.
  • Balanced BST (AVL, Red-Black) gives O(log n) search, insert and delete; an unbalanced BST degrades to O(n).

Data Structures Practice Questions with Answers

Attempt each question first, then open the explanation. All 8 questions below are free to read and require no signup.

  1. Q1.Which data structure follows the LIFO principle?

    Easy
    • AQueue
    • BStack
    • CLinked list
    • DTree
    +Show Answer & Explanation

    Answer: B. Stack

    Explanation: A stack removes the most recently added element first — Last In, First Out.

    Data Structures question 1 of 8

  2. Q2.The time complexity of binary search on a sorted array of n elements is:

    Easy
    • AO(n)
    • BO(log n)
    • CO(n log n)
    • DO(1)
    +Show Answer & Explanation

    Answer: B. O(log n)

    Explanation: Each comparison halves the search space, giving logarithmic time.

    Data Structures question 2 of 8

  3. Q3.Which data structure is used for Breadth First Search of a graph?

    Moderate
    • AStack
    • BQueue
    • CHeap
    • DTrie
    +Show Answer & Explanation

    Answer: B. Queue

    Explanation: BFS explores level by level, which requires FIFO ordering — hence a queue. DFS uses a stack (or recursion).

    Data Structures question 3 of 8

  4. Q4.The worst-case time complexity of searching in an unbalanced binary search tree is:

    Moderate
    • AO(1)
    • BO(log n)
    • CO(n)
    • DO(n²)
    +Show Answer & Explanation

    Answer: C. O(n)

    Explanation: If insertions arrive in sorted order the tree degenerates into a linked list, making search linear.

    Data Structures question 4 of 8

  5. Q5.Which operation is O(1) in an array but O(n) in a singly linked list?

    Moderate
    • AInsertion at head
    • BAccess by index
    • CDeletion at head
    • DTraversal
    +Show Answer & Explanation

    Answer: B. Access by index

    Explanation: Arrays support constant-time random access; a linked list must walk from the head to reach index i.

    Data Structures question 5 of 8

  6. Q6.A min-heap guarantees that:

    Moderate
    • AThe tree is sorted in-order
    • BThe root is the smallest element
    • CAll leaves are at the same level
    • DSearch takes O(log n)
    +Show Answer & Explanation

    Answer: B. The root is the smallest element

    Explanation: The heap property makes every parent ≤ its children, so the minimum sits at the root. Arbitrary search is still O(n).

    Data Structures question 6 of 8

  7. Q7.Which sorting algorithm has the best average-case time complexity?

    Easy
    • ABubble sort
    • BQuick sort
    • CSelection sort
    • DInsertion sort
    +Show Answer & Explanation

    Answer: B. Quick sort

    Explanation: Quick sort averages O(n log n); the other three average O(n²).

    Data Structures question 7 of 8

  8. Q8.A circular queue is preferred over a linear queue because it:

    Difficult
    • AUses less memory per element
    • BReuses vacated space at the front
    • CAllows random access
    • DSorts elements automatically
    +Show Answer & Explanation

    Answer: B. Reuses vacated space at the front

    Explanation: In a linear array queue the front space is wasted after dequeuing. A circular queue wraps the rear index around to reuse it.

    Data Structures question 8 of 8

Data Structures — Frequently Asked Questions

When should I use a linked list instead of an array?+

When insertions and deletions dominate and you do not need random access — a linked list avoids the O(n) shifting an array requires. If you index frequently or care about cache locality, an array wins.

What is the time complexity of a hash table lookup?+

O(1) on average with a good hash function and load factor. In the worst case, when all keys collide into one bucket, it degrades to O(n) — or O(log n) if the bucket uses a balanced tree.

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