stacks n' queues

  --add things
  --remove things
  --is it empty
  --size
  --(borderline) peek; allowed on homework, not on midterm
  
queue -- fifo: first in, first out behavior
  -think of it like a line at the grocery store
  -first in is first served, no cutting!

stack -- lifo: last in, first out behavior
  -for example, a stack of cafeteria trays
    -you don't want the bottom tray! and even if you did...
     good luck getting to it