CS62 - Spring 2021

CS62 - Spring 2021 - Class 10

Example code in this lecture

Lecture notes

administrative
   - Assignment 4
   - midterm in lab next Wednesday
      - review next Tuesday
      - can bring one page of notes (double-sided)
      - will post practice problems with solutions
         - study first!
         - then try the practice problems
         - try hard not to look at the solutions
         - find other ways to check your answers, like running your programs

Linear structures
   - similar to lists in that there is a sequential nature to the data
   - unlike lists, though, we can only add and remove items, but cannot access items by index or iterate through the items
   - look at Linear interface in StacksAndQueues code
      - we can:
         - add items
         - remove an item (and return it)
         - take a look at the next item
         - see if we have items left
      - notice that we don't have any notion of an index

stacks
   - Last In First Out (LIFO)
   - two basic operations: push and pop
      - push adds another item on to the top of the stack
      - pop removes the item on the top of the stack
   - think about a stack of plates at a buffet. The last plate to be put on top will be the first plate to be removed.
      - stacks usually grow up
      - everything happens on the top

   - look at the Stack interface in StacksAndQueues code
      - push
      - pop
      - peek (when we just want to see what's on top, but don't want to modify the stack)
      - empty

   - how could we implement this?
      - Linked list
         - always just manipulate the head of the beginning of the linked list
         - to push an item, just add it to the front
         - to remove an item, remove it from the front
         - look at LinkedStack in StacksAndQueues code
         - singly linked or doubly linked?
         - runtime of the different operations:
            - push: O(1)
            - pop: O(1)
            - peek: O(1)
            - empty: O(1)
      - ArrayList
         - where should the top of the stack go?
            - remember we'll be adding and deleting at the top of the stack
            - put the top of the stack at the END of the ArrayList
         - to push an item, add it to the end of the ArrayList
         - to remove an item, remove it from the end of the ArrayList
         - look at ArrayListQueue in StacksAndQueues code
         - runtime of the different operations:
            - push: O(1) (amortized)
            - pop: O(1)
            - peek: O(1)
            - empty: O(1)

      - which is better?
         - ArrayList is "amortized" O(1) run-time, however, any individual push operation could be O(n)
         - Memory trade-off is less clear
            - ArrayList could have lots of "open" memory
            - LinkedList has an extra reference for each data item

queues
   - if you're from the UK, you call a "line" (like waiting in line) a queue
   - First In First Out (FIFO)
   - two basic operations: enqueue and dequeue
      - enqueue adds another item on the the end of the queue
      - dequeue removes an item from the front of the queue
   - notice that like a stack the only way we manipulate the data is by adding and removing items, the difference is where we add and remove the items

   - look at the Queue interface in StacksAndQueues code
      - enqueue
      - dequeue
      - peek (when we just want to see what's on top, but don't want to modify the stack)
      - empty

   - how could we implement this?
      - Linked list
         - where should we add and remove items?
            - singly linked list with tail reference:
               - add to the back
               - remove from the front
            - doubly linked list with tail pointer: doesn't matter :)
         - look at LinkedQueue in StacksAndQueues code
         - runtime: O(1)

      - ArrayList based
         - where should we add and remove the items?
            - if we add them to the back, then the remove (dequeue) operation is going to be expensive
            - if we add them to the front, then the add (enqueue) operation is going to be expensive
         - we'll add at the back again and remove them from the front
         - look at LinkedQueue in StacksAndQueues code
         - runtime of different operations:
            - enqueue: O(1)
            - dequeue: O(n)
            - peek: O(1)
            - empty: O(1)
         - Even though both implement the List interface, it makes a difference which underlying data structure we use... pick the right one!

look at StackVQueue class in StacksAndQueues code
   - what will testStack and testQueue print out?
      - testStack
         daikon
         carrot
         banana
         apple

         (LIFO: last in first out)
      - testQueue
         apple
         banana
         carrot
         daikon

         (FIFO: first in first out)
   - What types of objects can we pass to testStack?
      - Anything that implements the Stack interface (ArrayListStack or LinkedStack)
   - Will the answer change depending on what we pass in?
      - Not if we've implemented them correctly :)
      - They both are stacks, just different ways of representing them
   - What types of objects can we pass to testLinear?
      - Anything that implements the Linear interface
      - All of our stacks and queue implement this interface
      - this allows us to pass either a stack OR a queue!

Applications
   - What are stacks and queues good for?

   - stacks
      - run-time (or call) stack example
         - we can write a simple method sum and follow it through the call-stack using the debugger
         public static int sum(int n){
            String temp = "Num: " + n;
            System.out.println(temp);

            if( n <= 1 ){
               return 1;
            }else{
               return n + sum(n-1);
            }
         }
      - searching
      - parsing (linguistics, code)

   - queues
      - scheduling tasks
         - which process should run next
      - modeling real world phenomena (lines show up in lots of places)
      - searching

Stacks/queues in Java:
   - dequeue interface: https://docs.oracle.com/javase/7/docs/api/java/util/Deque.html
   - Common classes that implement this interface:
      - ArrayDequeue
      - LinkedList