AdventOfCode/EnumerableExtensions.cs

namespace AdventOfCode;

public static class EnumerableExtensions
{
    public static IEnumerable<TSource> Flatten<TSource>(this TSource root, Func<TSource, IEnumerable<TSource>> flattener)
    {
        var toVisit = new Stack<TSource>();
        toVisit.Push(root);

        while (toVisit.TryPop(out var node))
        {
            yield return node;

            foreach (var child in flattener(node))
            {
                toVisit.Push(child);
            }
        }
    }

    public static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key, TValue defaultValue)
    {
        return dictionary.TryGetValue(key, out var value) ? value : defaultValue;
    }
}