PlacementPrepWhich data structure follows the Last In, First Out (LIFO) principle?
Solution
In a stack, the last element pushed is the first popped.
Answer: (C) Stack
Technical · CSE / IT
Data Structures MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
Which data structure follows the First In, First Out (FIFO) principle?
Solution
In a queue, elements leave in the order they arrived.
Answer: (D) Queue
What is the time complexity of accessing an element by index in an array?
Solution
The address is computed directly from the index.
Answer: (C) O(1)
What is the time complexity of searching in a balanced binary search tree?
Solution
Each comparison halves the remaining tree in a balanced BST.
Answer: (B) O(log n)
Which data structure is used to implement recursion?
Solution
Each function call is pushed onto the call stack.
Answer: (A) Stack
What is the in-order traversal of a binary search tree?
Solution
In-order (left, root, right) visits BST keys in sorted order.
Answer: (B) The keys in ascending order
Which traversal visits the root first, then the left subtree, then the right subtree?
Solution
Pre-order is root, left, right.
Answer: (B) Pre-order
What is the maximum number of nodes in a binary tree of height h (a single node has height 0)?
Solution
Level i can hold 2i nodes; summing levels 0 to h gives 2h+1 − 1.
Answer: (A) 2h+1 − 1
Which data structure is best for implementing a priority queue?
Solution
A heap gives O(log n) insertion and removal of the highest-priority element.
Answer: (C) Heap
What is the worst-case time to insert at the beginning of a singly linked list?
Solution
Only the head pointer and the new node's next pointer change.
Answer: (B) O(1)
In a hash table, what is it called when two keys map to the same index?
Solution
A collision occurs when two keys hash to the same slot; it is handled by chaining or open addressing.
Answer: (C) Collision
Which data structure is used in breadth-first search (BFS)?
Solution
BFS visits nodes level by level using a queue.
Answer: (B) Queue
Which data structure is used in depth-first search (DFS)?
Solution
DFS uses a stack (or recursion, which uses the call stack).
Answer: (C) Stack
A stack is empty. After push(1), push(2), pop(), push(3), push(4), pop(), what is on top?
Solution
Stack after each step: [1], [1,2], [1], [1,3], [1,3,4], [1,3]. Top is 3.
Answer: (A) 3
A queue is empty. After enqueue(1), enqueue(2), dequeue(), enqueue(3), dequeue(), what is at the front?
Solution
Queue: [1], [1,2], [2], [2,3], [3]. Front is 3.
Answer: (D) 3
What is the postfix form of the expression (A + B) * C?
Solution
First A + B becomes A B +, then multiply by C.
Answer: (B) A B + C *
How many edges does a tree with n nodes have?
Solution
A tree is connected and has no cycles, so it has exactly n − 1 edges.
Answer: (D) n − 1
Which of these is a non-linear data structure?
Solution
Trees and graphs are non-linear; arrays, stacks and queues are linear.
Answer: (C) Tree
What is the time complexity of deleting the minimum element from a min-heap of n elements?
Solution
The last element replaces the root and is sifted down the height of the heap.
Answer: (A) O(log n)
In a doubly linked list, each node contains:
Solution
Doubly linked list nodes have next and prev pointers.
Answer: (D) Data and pointers to both the next and previous nodes
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