Programming Languages

Currently, the most popular languages used in competitive programming are C++, Python, and Java. For example: In Google Code Jam 2017, among 3,000 participants, 79% used C++, 16% used Python, and 8% used Java.

Many people think C++ is the best choice for a competitive programmer, and C++ is available in almost all tournament systems. The benefits of C++ are that it is a highly efficient language and its standard library contains a large collection of data structures and algorithms.

On the other hand, it is a good idea to master different languages and understand their strengths. For example, if problems require very large integers, Python might be a better choice.

C++ Code Template

A typical C++ template for competitive programming looks like this:

#include <bits/stdc++.h>
using namespace std; 
int main() { 
// Solution goes here.
}

The #include line at the top of the code is a feature of the g++ compiler, allowing us to include the entire standard library. Thus, there is no need to include libraries separately like iostream, vector, and algorithm, as they are available automatically.

The using line declares that classes and functions of the standard library can be used directly in the code. Without this line we would have to write std::cout, but now it comes down to cout.

Input & Output

In contests, standard streams are used to read input and write output. In C++, standard streams are cin for input and cout for output. Additionally, the C functions scanf and printf can also be used.

The input for a program usually consists of numbers and strings that are separated by spaces or newlines. They can be read from the input stream cin like this:

int a,b; // This is a number
string x; // This is a word
cin >> a >> b >> x;

This kind of code always works, assuming there is at least one space or newline between each element of the input.

The output stream cout is used to print the output like this:

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

Input and output commonly cause performance bottlenecks in the program. The following lines at the start of the code make input and output more efficient:

ios::sync_with_stdio(0); cin.tie(0);

Working with Numbers

Integers

The most commonly used integer type in competitive programming is int, which is a 32-bit data type, covering ranges from $-2^{31}$ to $2^{31}-1$. If the int data type is not enough, the 64-bit long long type can be used.

long long x = 123456789123456789LL;

The LL suffix denotes that the number type is long long.

A common error when using the long long data type is that the int type is still used somewhere in the code. For example, the following code contains an overflow error:

int a = 123456789; 
long long b = a*a; 
cout << b << "\n"; // -1757895751

Even when variable b is of type long long, both numbers in the expression a*a are of type int, and the result is also of type int. Because of this, variable b will contain a wrong result. The problem can be solved by changing the type of a to long long or casting the expression to (long long)a*a.

Floating Point Numbers

Floating-point number types in competitive programming are the 64-bit double and its extension in the g++ compiler, the 80-bit long double. In most cases using a double is enough.

printf("%.9f\n", x);

Introduction to Competitive Programming in C++