Unique ID Firebase Database Help Please

Hi Everyone

I have to make an application

in that i have to make a unique id(Only Number) for individual user which don’t repeats

but I m little bit confused in making it

these are the blocks which i had made but after that i m confused what to do with them and i need unique id which dont repeats

It will get tag list and values from firebase database

like in this step i m confused that what should i do that i must make that id unique and dont repeat it
and after saving it will delete that number from list

I know that my english is bad
But u can understand it

Please Help Urgent

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