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C Programming with DSA Interview Questions in Gurgaon

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

C Programming with DSA jobs in Gurgaon

Gurgaon (Gurugram) is home to many multinational, consulting, fintech and consumer-internet offices. Interviews often expect strong business understanding along with technical skills, especially for analytics and product-facing roles.

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

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01

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.
02

How are graphs represented in C?

Trees and GraphsMedium
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An adjacency matrix is a V by V array where cell [u][v] marks an edge; it gives O(1) edge checks but uses O(V squared) memory, suiting dense graphs. An adjacency list stores for each vertex a list or dynamic array of neighbours, using O(V + E) memory, which suits sparse real world graphs like road or social networks. Weighted graphs store the weight with each edge.
03

You must schedule the maximum number of non-overlapping meetings in one room. What algorithm do you use?

Sorting, Searching and Dynamic ProgrammingMedium
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This is activity selection, solved greedily. Sort meetings by end time, then pick each meeting whose start is not earlier than the end of the last picked one. Choosing the earliest finishing meeting always leaves the most room for the rest, which can be proven by an exchange argument. Sorting dominates, giving O(n log n) time.
04

What is a double pointer and when do you need it?

Pointers and Memory ManagementMedium
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A double pointer, such as int **pp, stores the address of a pointer. It is needed when a function must change the caller's pointer itself, for example inserting at the head of a linked list or allocating memory inside a function and returning it through a parameter. It is also used for dynamic 2D arrays and arrays of strings, like char **argv.
05

Insert a value into a binary search tree recursively in C.

Trees and GraphsEasy
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Go left or right until an empty spot is found. typedef struct TNode { int key; struct TNode *left, *right; } TNode; TNode *insert(TNode *root, int key) { if (root == NULL) { TNode *n = malloc(sizeof *n); n->key = key; n->left = n->right = NULL; return n; } if (key < root->key) root->left = insert(root->left, key); else if (key > root->key) root->right = insert(root->right, key); return root; } Duplicates are ignored here.
06

A doubly linked list delete function leaves the list broken when deleting the head. What is missing?

Linked ListsMedium
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When deleting the head there is no previous node, so the head pointer itself must move to head->next, and the new head's prev must be set to NULL. Similarly, deleting the tail needs the new tail's next set to NULL. The fix is to check node->prev and node->next for NULL and update head or tail pointers accordingly, or use a sentinel node to avoid these cases.
07

What is a prefix sum array and why is it useful?

Arrays and StringsMedium
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A prefix sum array stores pre[i] = sum of elements from index 0 to i-1. Then the sum of any range from l to r is pre[r + 1] - pre[l] in O(1) after O(n) preprocessing. It is used for range sum queries, counting subarrays with a given sum together with a hash map, and 2D versions for matrix region sums.
08

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.
09

What is the difference between a macro and an inline function?

C Basics, Data Types and OperatorsMedium
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A macro is plain text substitution by the preprocessor, with no type checking, and arguments may be evaluated more than once, so SQUARE(i++) misbehaves. An inline function is a real function with type checks and single evaluation of arguments, which the compiler may expand in place for speed. Prefer static inline functions and use macros only for things like conditional compilation.
10

What is a function pointer and where is it used?

Pointers and Memory ManagementMedium
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A function pointer stores the address of a function, declared like int (*op)(int, int) = add;, and called with op(2, 3). It enables callbacks such as the comparator in qsort, dispatch tables replacing long switch statements, plugin interfaces and state machines in embedded code. Using typedef makes function pointer types much more readable.
11

Explain pointer arithmetic in C.

Pointers and Memory ManagementMedium
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Adding an integer to a pointer moves it by that many elements, not bytes, so p + 1 on an int pointer advances by sizeof(int). Subtracting two pointers into the same array gives the number of elements between them as ptrdiff_t. Arithmetic is valid only within an array or one past its end; going further is undefined behaviour. Void pointer arithmetic is not allowed in standard C.
12

Write a simple fixed-size memory pool allocator in C.

Pointers and Memory ManagementHard
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A free list of equal blocks avoids malloc in hot or embedded paths. #define BLOCKS 64 typedef union Block { union Block *next; char data[32]; } Block; static Block pool[BLOCKS]; static Block *freeList; void poolInit(void) { for (int i = 0; i < BLOCKS - 1; i++) pool[i].next = &pool[i + 1]; pool[BLOCKS - 1].next = NULL; freeList = pool; } void *poolAlloc(void) { Block *b = freeList; if (b) freeList = b->next; return b; } void poolFree(void *p) { Block *b = p; b->next = freeList; freeList = b; }
13

Tell me about a time you explained a complex data structure to your team.

Linked ListsMedium
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Situation: our team needed an LRU cache in a C service and several members had not built one. Task: align everyone before coding. Action: I drew the hash table and doubly linked list on a whiteboard, walked through get and put with a small example and shared a short test suite covering edge cases. Result: the team implemented it in two days with no major review issues.
14

What is the difference between stack and heap allocation?

Pointers and Memory ManagementEasy
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Stack memory holds local variables, is allocated and freed automatically when functions enter and return, and is very fast but limited in size, often 1 to 8 MB. Heap memory is allocated with malloc and freed manually, can be large and outlive the function, but is slower and can fragment or leak. Large or variable sized data and data that must outlive a function belong on the heap.
15

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.
16

A program that writes logs with fprintf loses the last lines when it crashes. Why, and how do you fix it?

Structures, Unions and File HandlingMedium
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stdio streams are buffered, so fprintf stores data in memory and writes it to disk only when the buffer fills, the file closes or fflush is called. A crash loses whatever is still in the buffer. Call fflush after important log lines, set line buffering with setvbuf(fp, NULL, _IOLBF, 0) or write critical messages to unbuffered stderr. For durability on disk, add fsync on POSIX systems.
17

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.
18

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.
19

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); }
20

How do you handle a coding interview problem when you are stuck?

Sorting, Searching and Dynamic ProgrammingMedium
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Situation: in an onsite round I could not see how to solve an interval problem. Task: still show strong problem solving. Action: I stated a brute force solution with its complexity, tried small examples aloud, noticed that sorting by start time helped and asked a clarifying question about overlaps. Result: I reached an O(n log n) solution with a small hint, and the feedback praised my communication.
21

A search on a sorted array of 10 lakh elements is done with a linear loop. How would you justify changing it?

Sorting, Searching and Dynamic ProgrammingEasy
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Linear search takes up to 10 lakh comparisons, while binary search needs about 20, because log base 2 of 10 lakh is roughly 20. If searches are frequent, this is a huge saving at zero memory cost. If data changes often, keeping it sorted costs inserts, so consider a balanced tree or hash table. Measure with realistic workloads before and after.
22

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.
23

What is a heap and how does it implement a priority queue?

Stacks, Queues and HashingMedium
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A binary heap is a complete binary tree stored in an array where every parent is smaller than its children for a min heap, or larger for a max heap. For index i, children are at 2i + 1 and 2i + 2. Insert adds at the end and sifts up, and extracting the top swaps it with the last element and sifts down, both O(log n). Building a heap from an array takes O(n).
24

Solve the 0/1 knapsack problem in C with a 1D DP array.

Sorting, Searching and Dynamic ProgrammingHard
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Iterate capacity backwards so each item is used once. int knapsack(const int *wt, const int *val, int n, int W) { int dp[W + 1]; for (int c = 0; c <= W; c++) dp[c] = 0; for (int i = 0; i < n; i++) { for (int c = W; c >= wt[i]; c--) { int take = dp[c - wt[i]] + val[i]; if (take > dp[c]) dp[c] = take; } } return dp[W]; } Time O(nW), space O(W).
25

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.
26

Find the next greater element for every array element using a monotonic stack.

Stacks, Queues and HashingMedium
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Keep indexes of elements still waiting for a greater value. void nextGreater(const int *a, int n, int *res) { int st[n], top = -1; for (int i = 0; i < n; i++) { res[i] = -1; while (top >= 0 && a[st[top]] < a[i]) { res[st[top--]] = a[i]; } st[++top] = i; } } For 4 5 2 25 the result is 5 25 25 -1, in O(n) time.
27

How do you check whether a linked list is a palindrome in O(1) extra space?

Linked ListsMedium
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Find the middle with slow and fast pointers, reverse the second half in place, then compare the first half and the reversed second half node by node. Finally reverse the second half again to restore the list, which interviewers appreciate. Time is O(n) and extra space is O(1). A simpler O(n) space approach copies values into an array or stack first.
28

How do you keep improving your DSA skills while working full time?

Sorting, Searching and Dynamic ProgrammingEasy
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Situation: after joining a services company I had little time for practice. Task: stay ready for product company interviews. Action: I set a goal of five problems a week on weekday mornings, joined weekend contests and kept a notebook of patterns and mistakes, revising it monthly. Result: within a year I reached a solid contest rating and cleared interviews for a backend role.
29

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.
30

You need to update one record in the middle of a large binary file without rewriting it all. How?

Structures, Unions and File HandlingMedium
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Use fixed-size records so each one starts at index times record size. Open the file in r+b mode, fseek to that offset, fwrite the new record and fflush. Read it back to verify if needed. For crash safety, write to a temporary file and rename, or keep a simple journal. Databases like SQLite solve this robustly if requirements grow.
31

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.
32

What is amortised analysis? Give an example.

Sorting, Searching and Dynamic ProgrammingHard
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Amortised analysis averages the cost of operations over a sequence, so occasional expensive steps are spread over many cheap ones. A dynamic array that doubles when full copies all elements on resize, which is O(n), but because doubling happens rarely, n appends cost O(n) in total, so each append is O(1) amortised. Union-find with path compression is another classic example.
33

Write a C program that counts lines, words and characters in a file.

Structures, Unions and File HandlingMedium
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Read character by character with fgetc. FILE *fp = fopen(path, "r"); if (!fp) { perror("fopen"); return 1; } long lines = 0, words = 0, chars = 0; int c, inWord = 0; while ((c = fgetc(fp)) != EOF) { chars++; if (c == '\n') lines++; if (isspace(c)) inWord = 0; else if (!inWord) { inWord = 1; words++; } } fclose(fp); printf("%ld %ld %ld\n", lines, words, chars); c is int so EOF can be distinguished from valid bytes.
34

How do you convert an infix expression to postfix?

Stacks, Queues and HashingMedium
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Use the shunting yard algorithm with an operator stack. Output operands directly. For an operator, pop operators of higher precedence, or equal precedence if left associative, to the output, then push it. Push '(' and on ')' pop until the matching '('. At the end, pop all remaining operators. For example a + b * c becomes a b c * +. Postfix is easy to evaluate with a stack.
35

What is the difference between strcpy, strncpy and snprintf for copying strings?

Arrays and StringsMedium
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strcpy copies until the null terminator with no bounds check, so it can overflow. strncpy limits the count but does not add a terminator if the source is too long and pads with zeros otherwise, which surprises many developers. snprintf(dst, size, "%s", src) always terminates and returns the length it wanted, so you can detect truncation. Prefer snprintf or a checked helper.
36

How do you explain the time complexity of your solution to an interviewer?

Trees and GraphsEasy
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Situation: in a technical round I solved a tree problem but the interviewer questioned its efficiency. Task: justify the complexity clearly. Action: I explained that each node is visited once and does constant work, so time is O(n), and that recursion depth equals tree height, giving O(h) space, worst case O(n) for a skewed tree. Result: the interviewer agreed and moved to a harder follow-up.

Companies with offices in Gurgaon (2)

Telecom

Ericsson

C Programming with DSA interview questions

  • 30 questions
  • 4 rounds

Gurugram, Bengaluru, Chennai +2

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Telecom

Nokia

C Programming with DSA interview questions

  • 30 questions
  • 4 rounds

Bengaluru, Chennai, Noida +1

View questions

Frequently asked questions

Which companies hire C Programming with DSA professionals in Gurgaon?

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

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

The questions depend more on the company than the city. Gurgaon's mix of employers decides the style: Interviews often expect strong business understanding along with technical skills, especially for analytics and product-facing roles

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

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

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