Learn DSA in C++ from scratch

01

Variables and types

Choosing int or long long changes whether a sum can overflow.

#include <iostream>
using namespace std;
int main() {
    int count = 3;
    cout << "items " << count << endl;
}

Output: items 3

02

Conditionals and loops

Array scans and graph traversals are built from branches and iteration.

#include <iostream>
using namespace std;
int main() {
    int total = 0;
    for (int value : {2, 4, 5}) if (value % 2 == 0) total += value;
    cout << total << endl;
}

Output: 6

03

Functions

Functions make an algorithm's inputs and output visible at the call site.

#include <iostream>
using namespace std;
int doubleValue(int value) { return value * 2; }
int main() { cout << doubleValue(6) << endl; }

Output: 12

04

Recursion

Recursive functions express tree traversal and divide-and-conquer directly.

#include <iostream>
using namespace std;
void countdown(int n) { if (n == 0) return; cout << n << endl; countdown(n - 1); }
int main() { countdown(3); }

Output: 3 2 1

05

Arrays and lists

vector is the resizable indexed container used for most sequence problems.

#include <iostream>
#include <vector>
using namespace std;
int main() { vector<int> numbers{3, 1, 2}; numbers.push_back(4); cout << numbers[1] << " " << numbers.size() << endl; }

Output: 1 4

06

Strings

std::string supports indexing and comparison for parsing and two-pointer problems.

#include <algorithm>
#include <iostream>
#include <string>
using namespace std;
int main() { string word = "level"; string copy = word; reverse(copy.begin(), copy.end()); cout << (word == copy) << endl; }

Output: 1

07

Hash maps and sets

Unordered containers provide average constant-time frequency and membership checks.

#include <iostream>
#include <unordered_set>
#include <unordered_map>
using namespace std;
int main() { unordered_set<int> seen{2, 4}; unordered_map<char, int> counts{{'a', 2}}; cout << seen.count(4) << " " << counts['a'] << endl; }

Output: 1 2

08

References and copying

A reference aliases an object; copy when a function or branch needs independent storage.

#include <iostream>
#include <vector>
using namespace std;
int main() { vector<int> first{1, 2}; vector<int> second = first; second[0] = 9; cout << first[0] << " " << second[0] << endl; }

Output: 1 9

09

Classes or structs

A struct is a compact node model for linked lists and trees.

#include <iostream>
using namespace std;
struct Node { int value; };
int main() { Node node{7}; cout << node.value << endl; }

Output: 7

10

Sorting with a custom comparator

A lambda lets you sort pairs and records by the field an algorithm needs.

#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;
int main() { vector<pair<char, int>> items{{'b', 2}, {'a', 1}}; sort(items.begin(), items.end(), [](auto a, auto b) { return a.second < b.second; }); cout << items[0].first << endl; }

Output: a

11

Reading input

cin reads judge input, while sync_with_stdio and tie keep standard input fast.

#include <iostream>
using namespace std;
int main() { ios::sync_with_stdio(false); cin.tie(nullptr); int a, b; cin >> a >> b; cout << a + b << endl; }

Input: 2 4

Output: 6