using System;
using System.Collections.Generic;

public class FastUniqueList<T>
{
    private readonly List<T> list = [];
    private readonly HashSet<T> set = [];

    public int Count => list.Count;

    public T this[int index] => list[index];

    public IEnumerator<T> GetEnumerator() => list.GetEnumerator();

    public bool Contains(T item)
    {
        return set.Contains(item);
    }

    public bool Add(T item)
    {
        if (!set.Add(item))
        {
            return false;
        }

        list.Add(item);
        return true;
    }

    public bool Remove(T item)
    {
        if (!set.Remove(item))
        {
            return false;
        }

        int index = list.IndexOf(item);
        int last = list.Count - 1;

        list[index] = list[last];
        list.RemoveAt(last);

        return true;
    }

    public void ForEach(Action<T> action)
    {
        list.ForEach(action);
    }
}