SpawnPoints and rejoining a room

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In my game I have 4 fixed SpawnPoints for up to 4 Players. I create a custom room Property in OnCreatedRoom()
for every spawnpoint and give it the value true (for free).

Whenever the OnJoinedRoom() callback is fired I loop through the properties to find the first free spawnpoint and set the corresponding room property to false (for occupied)

This works fine so far as every Player gets a different Spawnpoints.

Now I'd like to set the room property for the Spawnpoint to true again if the Player that occupies the Spawnpoint leaves the room. So if he or someone else enters the room again he can take that free Spawnpoint.

But how could I achieve this? Any suggestions would be appreciated.