title: Big O notation for Array/Stack/Queue
body: <p>I am wondering, why Big O notation is O(1) for Array/Stack/Queue in avg. cases when we are inserting and deleting an element ? </p>
  
  <p>In my understanding, it is O(1) because interting and deleting an element takes a constant amount of time no matter the amount of data in the set but I am still little bit confused. Any help will be highly appreciated in removing my confusion.</p>