Competitive programming with C++: setup and time limits

What competitive programming is

In competitive programming (CP) you solve well-defined problems by writing a program that reads input and prints output. An online judge runs your program on hidden tests and replies with a verdict: Accepted (AC), Wrong Answer (WA), Time Limit Exceeded (TLE), Runtime Error (RE) or Memory Limit Exceeded (MLE). Many Indian companies use the same style of problems in online assessments, so CP practice doubles as placement preparation.

Where to practise

A contest template

#include <bits/stdc++.h>     // GCC-only header that includes the whole standard library
using namespace std;
using ll = long long;

void solve() {
    int n;
    cin >> n;
    vector<ll> a(n);
    for (auto& x : a) cin >> x;
    cout << accumulate(a.begin(), a.end(), 0LL) << '\n';
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t = 1;
    cin >> t;                    // many problems have several test cases
    while (t--) solve();
}

Time limit maths

A rough rule: a judge machine does around 10^8 simple operations per second in C++ (it varies; treat it as an estimate). So from the input size n you can guess the complexity you need:

A good practice routine

1. Read the whole statement and the limits before coding. Work out the needed complexity.
2. Solve the sample by hand to make sure you understand it.
3. Think of edge cases: n = 1, all equal values, the largest values (overflow?), negative numbers.
4. Code, test on samples, then on your own tricky cases.
5. After the contest, read the editorial for problems you could not solve, and solve them again ("upsolving"). This is where most learning happens.