C# cheat and super-easy solution
Disclaimer: This is alternative solution and should be used only if other approaches are well-known.\nSince we know the range of input numbers (0 - 1000000) and
Disclaimer: This is alternative solution and should be used only if other approaches are well-known.\nSince we know the range of input numbers (0 - 1000000) and we have no memory constraints we can use predefined array of booleans to identify if the number is added.\n\n```c#\npublic class MyHashSet {\n    private bool[] arr = new bool[1000001];\n        \n    /** Initialize your data structure here. */\n    public MyHashSet() {\n        \n    }\n    \n    public void Add(int key) {\n        arr[key] = true;\n    }\n    \n    public void Remove(int key) {\n        arr[key] = false;\n    }\n    \n    public bool Contains(int key) {\n        return arr[key];\n    }\n}\n```
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