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C Programming with DSA Interview Questions in Kolkata

36 practice questions with answers for C Programming with DSA interviews in Kolkata.

C Programming with DSA jobs in Kolkata

Kolkata has a steady IT-services and BFSI technology job market around Salt Lake Sector V and New Town. Many openings are with service companies and their delivery centres, with a focus on strong fundamentals.

Below are 36 questions that match the mix of service, product and domain employers hiring c developers in Kolkata.

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01

What is a circular linked list and where is it used?

Linked ListsEasy
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In a circular linked list the last node points back to the first, so there is no NULL end. You can start at any node and visit all nodes, and keeping only a tail pointer gives O(1) access to both ends. It is used in round-robin CPU scheduling, multiplayer turn order and circular buffers. Traversal loops must stop when they return to the starting node.
02

Why are self-balancing trees like AVL and red-black trees needed?

Trees and GraphsMedium
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A plain BST can degrade to a linked list with O(n) operations when data arrives in sorted order. AVL trees keep the height difference of subtrees at most one using rotations, giving faster lookups. Red-black trees use colour rules with looser balance, so inserts and deletes need fewer rotations. Both guarantee O(log n). Red-black trees are used in the Linux kernel, C++ std::map and Java TreeMap.
03

How do you set, clear and toggle a particular bit in C?

C Basics, Data Types and OperatorsMedium
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Use bit masks with shift operators. x |= (1u << n); // set bit n x &= ~(1u << n); // clear bit n x ^= (1u << n); // toggle bit n int bit = (x >> n) & 1u; // read bit n Use unsigned types so shifts are well defined. These operations are common in embedded code for configuring hardware registers.
04

Which gcc flags would you use to build and debug a C program safely?

C Basics, Data Types and OperatorsMedium
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A good development build is: gcc -std=c17 -Wall -Wextra -Wpedantic -g -O1 \ -fsanitize=address,undefined \ -o app main.c utils.c -Wall and -Wextra turn on useful warnings, -g adds debug symbols for gdb, and sanitizers catch memory errors and undefined behaviour at runtime. For release, drop sanitizers and use -O2.
05

Write C code to count the number of set bits in an integer.

C Basics, Data Types and OperatorsMedium
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Brian Kernighan's trick clears the lowest set bit each step. int countBits(unsigned int n) { int count = 0; while (n) { n &= n - 1; count++; } return count; } It runs once per set bit. GCC and Clang also offer __builtin_popcount(n), which uses a CPU instruction when available.
06

Memory use of a long running C service grows every hour. How do you investigate?

Pointers and Memory ManagementHard
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Run it under Valgrind or with AddressSanitizer and LeakSanitizer in a test environment and reproduce typical traffic, then read the leak report with allocation stack traces. In production, compare heap usage over time with tools like heaptrack. Check error paths that return early without free, lost pointers after realloc, and caches without size limits. Fix ownership and add a leak check to CI.
07

What is a linked list and how is it different from an array?

Linked ListsEasy
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A linked list is a chain of nodes where each node holds data and a pointer to the next node, allocated separately on the heap. Unlike an array, it grows easily and inserts or deletes in O(1) once you have the position, but it has O(n) access by index, extra memory per node for pointers and poor cache locality. Arrays win for random access and iteration speed.
08

Your linked list program crashes when deleting the last node. What do you check?

Linked ListsMedium
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Check that the previous node's next pointer is set to NULL after freeing the last node, that the tail pointer is updated if you keep one and that you do not access the node after free. Also handle the case of a one-node list where head itself must become NULL, which requires passing a pointer to the head pointer. Run the code under AddressSanitizer to find invalid accesses.
09

Why is a circular queue better than a simple array queue?

Stacks, Queues and HashingMedium
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In a simple array queue, the front moves forward as elements are removed, so the space before it is wasted and the queue appears full even when there is room. A circular queue wraps indexes using modulo, so rear = (rear + 1) % size reuses freed slots. You track fullness with a count or by keeping one slot empty to distinguish full from empty.
10

What is a hash table and how are collisions handled?

Stacks, Queues and HashingMedium
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A hash table maps keys to array slots using a hash function, giving O(1) average insert, search and delete. Collisions happen when two keys map to the same slot. Separate chaining stores a linked list per slot. Open addressing probes for another free slot using linear probing, quadratic probing or double hashing. Keep the load factor low and resize when it grows to maintain speed.
11

After calling realloc, the program sometimes crashes. What could be wrong?

Pointers and Memory ManagementMedium
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realloc may move the block and returns the new address, so old pointers to the block become invalid. Writing ptr = realloc(ptr, n) also leaks the original memory if realloc fails and returns NULL. Store the result in a temporary pointer, check for NULL, then assign. Make sure no other pointers still refer to the old location after a successful move.
12

What is backtracking? Give an example.

Sorting, Searching and Dynamic ProgrammingMedium
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Backtracking builds a solution step by step and abandons a path as soon as it cannot lead to a valid answer, then tries the next option. It explores a search tree with pruning. Examples are N-Queens, Sudoku, generating permutations and subsets, and maze path finding. Time can be exponential, so good pruning and ordering choices matter a lot.
13

Tell me about a time you fixed a memory leak.

Pointers and Memory ManagementMedium
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Situation: a C based telemetry agent grew by 200 MB a day on customer servers. Task: find and fix the leak quickly. Action: I reproduced it with recorded traffic under Valgrind and found that an error path returned early without freeing a parsed message buffer. I restructured the function with a single cleanup label and added a leak check job to CI. Result: memory stayed flat for weeks after the patch.
14

What is the difference between malloc, calloc, realloc and free?

Pointers and Memory ManagementEasy
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malloc(size) allocates uninitialised memory. calloc(n, size) allocates memory for n elements and sets all bytes to zero, and it checks n times size for overflow. realloc(ptr, size) resizes a block, possibly moving it, and returns the new address. free(ptr) releases memory back to the allocator. All allocators return NULL on failure, so always check the result.
15

What is the volatile qualifier used for in C?

C Basics, Data Types and OperatorsMedium
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volatile tells the compiler that a variable can change outside normal program flow, for example a hardware register, a variable changed in an interrupt handler or memory shared with another device. The compiler must read it from memory every time and cannot cache it in a register or remove repeated reads. It does not provide atomicity or thread safety on its own.
16

What is the difference between greedy algorithms and dynamic programming?

Sorting, Searching and Dynamic ProgrammingMedium
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A greedy algorithm makes the best local choice at each step and never revisits it, which works only when the greedy choice property holds, as in activity selection, Huffman coding and Dijkstra. Dynamic programming considers all relevant subproblem choices and combines them, so it works where greedy fails, such as 0/1 knapsack or coin change with arbitrary denominations. Greedy is faster when provably correct.
17

Implement union-find with path compression and union by rank.

Trees and GraphsHard
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Two small functions cover most uses. int parent[MAXN], rnk[MAXN]; void makeSet(int n) { for (int i = 0; i < n; i++) { parent[i] = i; rnk[i] = 0; } } int find(int x) { if (parent[x] != x) parent[x] = find(parent[x]); return parent[x]; } int unite(int a, int b) { a = find(a); b = find(b); if (a == b) return 0; if (rnk[a] < rnk[b]) { int t = a; a = b; b = t; } parent[b] = a; if (rnk[a] == rnk[b]) rnk[a]++; return 1; }
18

Write a C program to swap two numbers without using a third variable.

C Basics, Data Types and OperatorsEasy
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Using arithmetic or XOR through pointers: void swap(int *a, int *b) { if (a == b) return; *a = *a ^ *b; *b = *a ^ *b; *a = *a ^ *b; } The a == b check matters because XOR swapping a variable with itself sets it to zero. In real code a temporary variable is clearer and just as fast.
19

Evaluate a postfix expression of single digit numbers using a stack.

Stacks, Queues and HashingMedium
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Push digits and apply operators to the top two values. int evalPostfix(const char *s) { int st[100], top = -1; for (; *s; s++) { if (isdigit((unsigned char)*s)) st[++top] = *s - '0'; else if (strchr("+-*/", *s)) { int b = st[top--], a = st[top--]; switch (*s) { case '+': st[++top] = a + b; break; case '-': st[++top] = a - b; break; case '*': st[++top] = a * b; break; case '/': st[++top] = a / b; break; } } } return st[top]; } 23*5+ evaluates to 11.
20

Explain storage classes in C.

C Basics, Data Types and OperatorsMedium
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auto is the default for local variables, stored on the stack with block lifetime. register hints the variable should live in a CPU register, and its address cannot be taken. static gives a local variable lifetime for the whole program, or limits a global to its file. extern declares a variable defined in another file. Storage class decides scope, lifetime and linkage.
21

Explain Kadane's algorithm.

Arrays and StringsMedium
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Kadane's algorithm finds the maximum sum contiguous subarray in O(n). It keeps the best sum ending at the current index: cur = max(a[i], cur + a[i]), and updates the global best with best = max(best, cur). If the running sum becomes worse than starting fresh, it starts a new subarray. Initialise with the first element to handle arrays where all numbers are negative.
22

What is the difference between const int *p, int *const p and const int *const p?

C Basics, Data Types and OperatorsMedium
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const int *p is a pointer to a constant int: you cannot change the value through p, but p can point elsewhere. int *const p is a constant pointer: p always points to the same address, but the value can change. const int *const p fixes both. Read declarations from right to left to understand them. const pointers to data are common in function parameters.
23

Implement merge sort in C.

Sorting, Searching and Dynamic ProgrammingMedium
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Split, sort halves, merge with a temp buffer. void merge(int *a, int l, int m, int r, int *tmp) { int i = l, j = m + 1, k = l; while (i <= m && j <= r) tmp[k++] = (a[i] <= a[j]) ? a[i++] : a[j++]; while (i <= m) tmp[k++] = a[i++]; while (j <= r) tmp[k++] = a[j++]; for (k = l; k <= r; k++) a[k] = tmp[k]; } void mergeSort(int *a, int l, int r, int *tmp) { if (l >= r) return; int m = l + (r - l) / 2; mergeSort(a, l, m, tmp); mergeSort(a, m + 1, r, tmp); merge(a, l, m, r, tmp); }
24

Why is a dummy or sentinel node useful in linked list code?

Linked ListsMedium
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A dummy node placed before the real head removes special cases for an empty list or for inserting and deleting at the head, because every real node then has a previous node. Code for merging lists, removing elements and partitioning becomes shorter and less error prone. At the end, return dummy.next as the new head. It costs one extra node.
25

How does binary search work and what are common bugs?

Sorting, Searching and Dynamic ProgrammingEasy
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Binary search works on sorted data by comparing the middle element with the target and discarding half the range each step, giving O(log n). Common bugs are computing mid as (low + high) / 2, which can overflow, so use low + (high - low) / 2; wrong loop conditions causing infinite loops; and off-by-one errors when finding first or last occurrences. Test with zero, one and two elements.
26

Explain the memory layout of a C program.

C Basics, Data Types and OperatorsMedium
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From low to high address, a typical process has the text segment holding code, the initialised data segment for globals and statics with values, BSS for uninitialised or zero globals, the heap growing upward for malloc memory, and the stack growing downward for function frames and local variables. Knowing this helps explain stack overflow, memory leaks and where variables live.
27

Your program sorts 1 crore integers in the range 0 to 999 and qsort is too slow. What would you do?

Sorting, Searching and Dynamic ProgrammingMedium
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With a small known range, use counting sort: count occurrences in an array of 1000 counters, then rewrite the array in order. It runs in O(n + k) time with only 1000 extra integers, far faster than O(n log n) comparison sorts. For larger ranges, radix sort on bytes works well. qsort also pays a function call per comparison through its comparator pointer.
28

Merge two sorted linked lists into one sorted list.

Linked ListsMedium
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A dummy node keeps the code simple. Node *mergeSorted(Node *a, Node *b) { Node dummy = {0, NULL}; Node *tail = &dummy; while (a && b) { if (a->data <= b->data) { tail->next = a; a = a->next; } else { tail->next = b; b = b->next; } tail = tail->next; } tail->next = a ? a : b; return dummy.next; } It reuses existing nodes in O(n + m) time.
29

What is the difference between text and binary file modes?

Structures, Unions and File HandlingMedium
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In text mode, the C library may translate line endings, for example \r\n on Windows to \n, and a Ctrl+Z byte can mean end of file. Binary mode reads and writes bytes exactly as stored. Use text mode with fprintf and fgets for human readable files and binary mode with fread and fwrite for images, structs and any exact byte format. On Linux both behave the same.
30

What is structure padding and how can you reduce it?

Structures, Unions and File HandlingMedium
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The compiler inserts padding bytes between members so each one is aligned, which can make a struct larger than the sum of its members. struct { char a; int b; char c; } is usually 12 bytes instead of 6. Reduce padding by ordering members from largest to smallest alignment. Packing with __attribute__((packed)) removes padding but can slow access or fault on some CPUs.
31

Write a C function to count words in a sentence.

Arrays and StringsEasy
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Count transitions from space to non-space. #include <ctype.h> int countWords(const char *s) { int count = 0, inWord = 0; for (; *s; s++) { if (isspace((unsigned char)*s)) { inWord = 0; } else if (!inWord) { inWord = 1; count++; } } return count; } This handles multiple spaces, tabs and leading or trailing spaces correctly.
32

What is dynamic programming?

Sorting, Searching and Dynamic ProgrammingMedium
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Dynamic programming solves problems that have overlapping subproblems and optimal substructure by storing results of subproblems instead of recomputing them. Top-down memoisation caches recursive calls, while bottom-up tabulation fills a table iteratively. Classic examples are Fibonacci, 0/1 knapsack, longest common subsequence, coin change and edit distance. Defining the state and transition clearly is the hardest part.
33

Rotate an array to the right by k positions in O(n) time and O(1) space.

Arrays and StringsMedium
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Use three reversals. void rev(int *a, int l, int r) { while (l < r) { int t = a[l]; a[l++] = a[r]; a[r--] = t; } } void rotate(int *a, int n, int k) { k %= n; if (k == 0) return; rev(a, 0, n - 1); rev(a, 0, k - 1); rev(a, k, n - 1); } For 1 2 3 4 5 with k = 2 the result is 4 5 1 2 3.
34

Count the number of islands in a grid of land and water cells. What approach do you take?

Trees and GraphsMedium
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Treat each land cell as a graph node connected to its four neighbours. Scan the grid, and when you find unvisited land, increment the count and flood fill with DFS or BFS to mark the whole island visited. Time and space are O(rows times cols). For very large grids, use iterative BFS to avoid recursion depth issues, or union-find.
35

Tell me about a project where you worked with files or binary data in C.

Structures, Unions and File HandlingMedium
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Situation: in my final year project we logged GPS data on a microcontroller SD card. Task: store records reliably with small memory. Action: I designed a packed 16 byte record with fixed-width types, wrote it with a checksum, flushed every ten records and wrote a PC tool to decode the file. Result: we logged a full day with no corruption and could replay any trip.
36

What is typedef and how is it used with structures?

Structures, Unions and File HandlingEasy
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typedef creates an alias for a type. typedef struct Node { int data; struct Node *next; } Node; lets you write Node *head instead of struct Node *head. It is also useful for function pointer types and platform specific types. Some style guides avoid hiding pointers behind typedefs because it makes ownership and const usage less clear.

Companies with offices in Kolkata (2)

IT services

Wipro

C Programming with DSA interview questions

  • 30 questions
  • 4 rounds

Bengaluru, Hyderabad, Pune +2

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Telecom

Ericsson

C Programming with DSA interview questions

  • 30 questions
  • 4 rounds

Gurugram, Bengaluru, Chennai +2

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Frequently asked questions

Which companies hire C Programming with DSA professionals in Kolkata?

Companies with large offices in Kolkata include Wipro, Ericsson. Use their company pages below to practise employer-specific sets.

Are C Programming with DSA interviews in Kolkata different from other cities?

The questions depend more on the company than the city. Kolkata's mix of employers decides the style: Many openings are with service companies and their delivery centres, with a focus on strong fundamentals

How do I find C Programming with DSA jobs in Kolkata?

Shortlist companies with Kolkata offices, keep your CV focused on C Programming with DSA projects, apply on company career pages and job portals with the Kolkata filter, and practise the questions on this page before each interview.

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