Basics

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C++ code template

A typical C++ code template for competitive programming

    #include <bits/stdc++.h>
    using namespace std;

    #define int long long
    #define endl '\n'
    #define fastio() ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)

    int32_t main() {
        fastio();
        
        // Solution comes here

        return 0;
    }

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Input/Output (IO)

    int a, b;
    string x;
    cin >> a >> b >> x;

    int a = 123, b = 456;
    string x = "monkey";
    cout << a << " " << b << " " << x << "\n";

• Sometimes the program should read a whole line from the input, possibly containing spaces. This can be accomplished by using the getline function:

    string s;
    getline(cin, s);

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Working with numbers

1. Integers

   In competitive programming, the most common integer type is int (32-bit), with a range of about ±2×10⁹. For larger numbers, use long long (64-bit), which supports values up to ±9×10¹⁸.

   Be careful with mixed-type arithmetic — for example:

        int a = 123456789;
        long long b = a * a; // Wrong! Still computed as int
   
   
        long long b = (long long)a * a; // Correct
   

   Although __int128_t (128-bit) exists in g++, it’s rarely needed and not supported on all platforms. Usually, long long is sufficient for most contest problems.

2. Modular arithmetic

   We use x mod m to mean the remainder when x is divided by m. For example:

   In contests, when answers are very large, it’s common to output them modulo m (e.g., 10^9 + 7) to keep values manageable and avoid overflow.

   Key Properties: Mod can be safely applied before or after arithmetic operations:

   • (a + b) % m = ((a % m) + (b % m)) % m
   • (a - b) % m = ((a % m) - (b % m)) % m
   • (a * b) % m = ((a % m) * (b % m)) % m

   This ensures intermediate results stay small.

   Example: Factorial Modulo m

        long long x = 1;
        for (int i = 2; i <= n; i++) {
            x = (x * i) % m;
        }
        cout << x << "\n";
   

   Handling Negative Mods In C++ and many languages, a negative result can occur with subtraction. Fix it like this:

        x = x % m;
        if (x < 0) x += m;
   

   This ensures the result is always in the range 0 … m - 1

3. Floating point numbers

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4. Shortening code

   1. Type names

      To simplify long or complex type names, use the typedef keyword. For example, long long can be shortened to ll:

           typedef long long ll;
      
           ll a = 123456789;
           ll b = 987654321;
           cout << a * b << "\n";
      
           typedef vector<int> vi;
           typedef pair<int, int> pi;
                  
           vi nums = {1, 2, 3};
           pi point = {4, 5};
      

   2. Macros

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5. Mathematics

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