Advent of Code 2021

January 2, 2026 · View on GitHub

Info and problems are available at https://adventofcode.com/2021

Overview

Each day's solutions are in a subdirectory, 01, 02, etc.

Solutions are C++. Compile with:

g++ -std=c++20 -Wall -g -o doit doit.cc

There are occasional problems that need something special; in that case a comment at the start of the file will say what to do.

Example inputs, if any, are called input1, input2, etc. The real input, as downloaded from the AOC website, would be called input, though my inputs are not included in the repository.

Input is on stdin, output is printed to stdout. Run part 1 as ./doit 1 < input and part 2 as ./doit 2 < input

Sometimes I might go back and revisit a problem in a different (usually more efficient) way. Alternatives will be other .cc files starting with doit.

Here's my list of recommended problems for the year, along with direct links. Recommendations are based on a combination of factors: perhaps the problem admits an unusual and clever solution, or perhaps it would allow an interesting visualization, or perhaps the problem description itself was cute. In whatever way, I found the problem unusually fun. Each of these problems is either ⭐ (recommended) or ⭐⭐ (highly recommended).

Advent calendar for 2021