Lesson 01 · 12 min
C++ Setup, Hello & I/O
How a C++ program runs, iostream, cin/cout, competitive programming fast I/O.
Hello, C++
Every C++ program needs an entry point: int main(). When main returns 0, the OS treats it as success.
Minimal program
#include <bits/stdc++.h>
using namespace std;
int main() {
cout << "Hello CodeMentor\n";
return 0;
}#include <bits/stdc++.h> pulls almost all STL headers (common in contests). In production code, include only what you need (<iostream>, <vector>, …).
Reading & writing
| Goal | Code |
|---|---|
cout << x << " " << y << "\n"; | |
| Read one value | cin >> n; |
| Read until fail | while (cin >> x) |
Interview / LeetCode habit
On LeetCode you usually only write a class method — no main. Locally, CodeMentor wraps your method in a harness that feeds test input.
Fast I/O (contests)
ios::sync_with_stdio(false);
cin.tie(nullptr);Do this once at the start of main in contest programs. Skip it inside LeetCode class methods.
Rules to remember
- main must return int (usually 0).
- Prefer '\n' over endl (endl flushes — slower).
- cin >> skips whitespace; getline reads a full line.
- On LeetCode: implement the method, not main.
Check yourself
1. What does return 0; from main usually mean?
2. On LeetCode Two Sum, what do you write?