C Programming with DSA Interview Questions in Chennai

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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); }

Copy a file in C using fread and fwrite.

Structures, Unions and File HandlingMedium

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Use a buffer and binary mode. int copyFile(const char *src, const char *dst) { FILE *in = fopen(src, "rb"); if (!in) return -1; FILE *out = fopen(dst, "wb"); if (!out) { fclose(in); return -1; } char buf[8192]; size_t n; while ((n = fread(buf, 1, sizeof buf, in)) > 0) { if (fwrite(buf, 1, n, out) != n) break; } fclose(in); return fclose(out); }

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.

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

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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.

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.

How do you prioritise between writing fast code and readable code?

Linked ListsMedium

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Situation: a teammate replaced a clear linked list loop with a complex XOR trick to save memory. Task: decide what to keep. Action: I measured memory and speed, found the saving was under one percent and proposed keeping the readable version with a comment, while optimising a real hotspot the profiler showed. Result: we kept clear code and got a measurable 20 percent speedup elsewhere.

What is backtracking? Give an example.

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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.

Write BFS on a graph stored as an adjacency matrix.

Trees and GraphsMedium

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Use a queue and a visited array. void bfs(int n, int adj[n][n], int start) { int visited[n], queue[n], front = 0, rear = 0; for (int i = 0; i < n; i++) visited[i] = 0; visited[start] = 1; queue[rear++] = start; while (front < rear) { int u = queue[front++]; printf("%d ", u); for (int v = 0; v < n; v++) { if (adj[u][v] && !visited[v]) { visited[v] = 1; queue[rear++] = v; } } } }

A program prints garbage values for a local variable. What is the likely cause?

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Local automatic variables are not initialised in C, so they hold whatever bytes were on the stack. Reading them is undefined behaviour and gives random results. Initialise every variable at declaration, such as int sum = 0;. Compiling with -Wall -Wuninitialized and running under Valgrind or with -fsanitize=memory helps detect such reads early.

Allocate and free a dynamic 2D array of rows by cols in C.

Pointers and Memory ManagementMedium

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A single block is cache friendly and easy to free. int (*grid)[cols] = malloc(rows * sizeof *grid); if (!grid) return -1; grid[2][3] = 5; free(grid); // array of row pointers alternative int **m = malloc(rows * sizeof *m); for (int i = 0; i < rows; i++) m[i] = calloc(cols, sizeof **m); for (int i = 0; i < rows; i++) free(m[i]); free(m); The first form uses a variable length array type.

What is the difference between greedy algorithms and dynamic programming?

Sorting, Searching and Dynamic ProgrammingHard

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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.

What is amortised analysis? Give an example.

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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.

What are the basic data types in C and their typical sizes?

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C has char (1 byte), short (usually 2), int (usually 4), long (4 on Windows, 8 on 64-bit Linux), long long (at least 8), float (4) and double (8). The standard only fixes minimum ranges, so sizes depend on the platform. Use sizeof to check and the fixed-width types from stdint.h such as int32_t and uint8_t when exact sizes matter.

Explain the tree traversal orders.

Trees and GraphsEasy

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In-order visits left, node, right and gives sorted output for a BST. Pre-order visits node, left, right and is used to copy or serialise a tree. Post-order visits left, right, node and is used to delete a tree or evaluate expression trees. Level-order visits nodes level by level using a queue. The first three are depth first and use recursion or an explicit stack.

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

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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.

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.

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

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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.

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.

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

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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.

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

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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.

Implement your own version of memcpy.

Pointers and Memory ManagementHard

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Copy byte by byte through unsigned char pointers. void *my_memcpy(void *dest, const void *src, size_t n) { unsigned char *d = dest; const unsigned char *s = src; while (n--) { *d++ = *s++; } return dest; } memcpy must not be used for overlapping regions; memmove handles overlap by copying backwards when needed. Real libraries copy whole words for speed.

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

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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).

A strlen call crashes on a buffer received from a network packet. What is wrong?

Arrays and StringsMedium

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Network data is not guaranteed to be null terminated, so strlen keeps reading past the buffer until it hits a zero byte or invalid memory. Always track the received byte count and use length-aware functions such as memchr, strnlen or memcpy with explicit sizes. If you need a C string, allocate one extra byte and add the terminator yourself after checking the length.

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

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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.

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.

How do you convert an infix expression to postfix?

Sorting, Searching and Dynamic ProgrammingEasy

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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.

How does binary search work and what are common bugs?

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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.

Describe how you approach a coding problem you have never seen before.

Arrays and StringsMedium

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Situation: in a coding round I got an unfamiliar matrix problem. Task: solve it within 30 minutes. Action: I restated the problem, worked through a small example by hand, explained a brute force first, then spotted that a prefix sum would remove repeated work and coded it in C while talking through edge cases like empty rows. Result: I finished in 25 minutes and the interviewer praised my clear reasoning.

You need to look up 10 lakh user ids quickly in a C program. What structure do you choose?

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A hash table gives O(1) average lookup. Size it so the load factor stays under about 0.7, choose a good hash for the key type and use open addressing for better cache behaviour. If ids are dense integers in a known range, a plain array or bitmap is even faster. If the data is static, a sorted array with binary search is simple and memory efficient at O(log n).

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

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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.

What are the main stages of compiling a C program?

Structures, Unions and File HandlingEasy

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Preprocessing expands #include files and macros and removes comments. Compilation turns the preprocessed code into assembly. Assembly converts it into an object file of machine code. Linking combines object files with libraries such as libc to produce the executable. With gcc you can stop at each stage using -E, -S and -c, which helps when debugging macro or linker errors.

A file opened with fopen is NULL in production but fine on your machine. How do you debug?

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Always check the return value and print the reason with perror or strerror(errno), which shows errors like No such file or directory or Permission denied. Common causes are a relative path resolved from a different working directory, missing permissions for the service user, a full disk or too many open files. Use absolute or configured paths and close every file you open.

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.

What is the master theorem used for?

Sorting, Searching and Dynamic ProgrammingHard

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The master theorem gives the time complexity of divide and conquer recurrences of the form T(n) = aT(n/b) + f(n). Compare f(n) with n to the power log base b of a. For merge sort, a = 2, b = 2 and f(n) = n, which match, giving O(n log n). For binary search, a = 1 and b = 2 with constant work, giving O(log n). It does not cover every recurrence.

Explain the memory layout of a C program.