Learn Data Structures & Algorithms in C
C gives you arrays, pointers and a handful of library calls — and nothing else. This course teaches every structure the way you will write it in a C interview: a probing hash table, a binary heap, a queue in an array, linked lists and trees with malloc, all built once and reused, and every solution judged by GCC against hidden tests.
int binary_search(int a[], int n, int target) {
int lo = 0, hi = n - 1;
while (lo <= hi) {
int mid = lo + (hi - lo) / 2;
if (a[mid] == target) return mid;
if (a[mid] < target) lo = mid + 1;
else hi = mid - 1;
}
return -1;
}Why C for the DSA round
You build the structures, so you understand them
There is no hash map to import. The course writes one with you in Level 1, a heap in Level 2 and adjacency lists in Level 3, and every later lesson reuses the same 20-line pieces — which is exactly what an interviewer wants to see you do.
The landmines are taught, not discovered
Signed overflow is undefined, uninitialised memory is garbage, an out-of-bounds write is silent, strlen in a loop is O(n²), == compares addresses. Each one is taught at the point it bites, with the fix.
Nothing hides the cost
No library call is doing work you cannot see. The judge measures your run time and memory, and the scaling graph compares your solution with the optimal one.
The C you will actually use
Every structure is taught with the C tool that implements it — and what it costs.
- int *a, int n — arrays that know their length
- a growable array with realloc doubling
- an open-addressing hash table in a static array
- a binary heap in a plain array
- stack and queue as an array plus an index
- qsort with a safe comparator, hand-written binary search
- structs and malloc for lists, trees and tries
- long long, %lld and the overflow rules
A real editorial, in C
From the free Two Sum page: the one-pass hash-map approach, exactly as it appears in the course. Every snippet in every editorial was run against the judge’s own tests.
int* twoSum(int* nums, int numsSize, int target, int* returnSize) {
int cap = 1 << 18;
while (cap < numsSize * 2) cap <<= 1;
long long* keys = malloc(sizeof(long long) * cap);
int* idx = malloc(sizeof(int) * cap);
char* used = calloc(cap, 1);
int* out = malloc(sizeof(int) * 2);
*returnSize = 0;
for (int j = 0; j < numsSize; j++) {
long long need = (long long)target - nums[j];
unsigned long long h = (unsigned long long)need * 1315423911ULL & (cap - 1);
while (used[h]) { if (keys[h] == need) { out[0] = idx[h]; out[1] = j; *returnSize = 2; goto done; } h = (h + 1) & (cap - 1); }
{
long long x = nums[j];
unsigned long long g = (unsigned long long)x * 1315423911ULL & (cap - 1);
while (used[g] && keys[g] != x) g = (g + 1) & (cap - 1);
if (!used[g]) { used[g] = 1; keys[g] = x; idx[g] = j; }
}
}
done:
free(keys); free(idx); free(used);
return out;
}Free problems with C solutions
Full statement, examples, and a worked editorial with C code — no account needed to read, a free one to solve.
Seven levels, in order
Each level leans on the one below it. Browse every topic →
- L1Foundations & Linear StructuresThe base: complexity, the language bridge, and the linear structures every later level leans on.
- L2Non-linear StructuresTrees and forests — hierarchical and priority-driven data.
- L3GraphsModel anything as nodes and edges, then traverse and optimize.
- L4Algorithmic ParadigmsThe ways of thinking: greedy, divide & conquer, backtracking, and DP.
- L5Advanced & SpecializedThe hard tier — string algorithms and specialized structures.
- L6Interview Mastery CapstoneEverything under interview conditions: patterns, constraints, timed practice.
- L7Interview BankThe high-frequency questions — the ones asked again and again in real interviews.
C & DSA — questions people ask
Can I do DSA interviews in C?
Yes. C is accepted wherever you choose your language, and embedded, systems and firmware interviews often expect it. Interviewers in C care that you can build the structure yourself — a hash table, a heap, a queue — and that is what every lesson here trains.
Do I need to write input parsing?
No. Practice problems give you a function to complete, with arrays passed as a pointer and a length and results returned through a buffer, like a real platform. The lessons also show complete programs you can run and edit.
Which compiler does the judge use?
GCC with -std=c11 and -O2, with the maths library linked. Compiler built-ins such as __builtin_popcount are available.
What is free?
A free account opens the most-asked problems with full editorials forever, plus a 7-day trial of the starter topics. Full access opens all seven levels, mock interviews and the certificate — and the Python, Java, JavaScript and C++ courses too.
Start in C today
Free account, free problems in every level, a 7-day trial of the starter topics. Full access opens everything for a year — $49 a year (international price; the price for your region is shown at checkout).