CS100-Stacks & Queues Part 2
As you arrive, continue looking at the
non-recursive isAncestor problem from
last time. You’ll be seeing more
problems like it in class today.
What You Will Do Today
• You will solve more problems that involve
“deferred work”
• You will be able to explain how stacks and
queues differ in solving deferred work
problems
• You will be exposed to the specialized names
of functions in stacks (push, pop, peek) – you
need to memorize these
Similar Problem: Number Game
The number game starts with a particular number.
You win the game if you can reduce the number
to 0 using only the operations allowed in the
game. The operations allowed are:
Subtract 7
Subtract 9
Subtract 20
Write a function isWinnable to determine if the
game is winnable for a given starting value.
Number Game 2
The number game starts with a particular number. You
win the game if you can reduce the number to 0 using
only the operations allowed in the game. The
operations allowed are:
Subtract 3
Divide by 2 (only allowed if even)
Subtract 20
Write a function isWinnable to determine if the game is
winnable for a given starting value.
For this problem (and the other problems in today)
don’t use recursion
Number Game 3: Good Move
The number game starts with a particular number. You win
the game if you can reduce the number to 0 using only the
operations allowed in the game. The operations allowed
are:
Subtract 3
Subtract -1 (a.k.a. “add 1”)
Subtract 7
Write a function to determine a good move. A good move is
one that can win the game in the minimum number of
moves.
e.g. (20 can be -1+7+7+7 or 3+3+3+3+3+3+3+-1…or others)
But goodMove3(20) could return -1 or 7 but not 3, because
-1+7+7+7 is the shortest solution
Queue
• It’s when you add to the end of a list but remove
from the beginning
• Java does have a Queue interface, but in this class
we’ll just use a LinkedList
• Sometimes adding to a queue and removing from
a queue are called “enqueue” and “dequeue”
• Particularly useful when you want to do
something in the minimum number of steps,
because smaller step counts always happen
earlier
Number Game 2: States List
Subtract 3
Divide by 2 (only allowed if even)
Subtract 20
Write a function statesList2 that returns a list of game states
that win the game in a minimum number of moves.
e.g. 32’s state list looks like [32, 12, 6, 3, 0].
Hint: you’ll want to keep the list of states in your “to do”,
rather than nums.
Challenge: write MovesList that returns the winning moves
instead (e.g. [20,2,2,3] for 32)
After completing this, please submit your code via ambient.
Stack
• A stack is a list that you add to the beginning and
remove from the beginning. We call the beginning the
“top” of the stack
• Java has a stack class, which we will use (although we
could really just use a list)
• Stacks have special names:
– push(element) puts a new element on the top of the stack
– pop() removes an element from the top of the stack
– peek() gets the element at the top of the stack without
removing it
• Often useful in “tree like” problems where things have
subthings, which may have still more sub things
Matching Parens
• You’ve got a string consisting only of ‘(‘ and ‘)’
and ‘[‘ and ‘]’
• You want to know of the parents “match”.
That is, every opening paren has a closing
paren of the same type, and they are in the
right order. For example:
• “()” “([])” “[()]” “([]())” “()[]” match
• “)” “(]” “)(“ “()()(“ “((())” do not match