View state tells you about the page size at run time. 135KB is allowed on VF page, if your page exceed that, you get an error saying view state exceed. Your class variables contribute in view state
You can use transient keyword for all your controller properties which you only need one transaction. Like if you want to only display a list of records, but don't want that in next action. You can keep it transient.
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In an interview they asked question like how will you reduce the view state size ?
thank you so much