reversing a linked list in ruby using a stack - ruby

Question

I am implementing a reversed linked list using a stack.
Class Stack has methods push and pop, which turn a linked list into a stack from which the last element can be extracted in order to reverse the order.
I've encountered a similar situation here, and I'm trying to add a method to that base case. I'm trying to implement a reverse_list method that makes use of the Stack class's push and pop in order to change the pointer and reverse the linked list.
This is what I tried:
class LinkedListNode
attr_accessor :value, :next_node
def initialize(value, next_node=nil)
#value = value
#next_node = next_node
end
end
def print_values(list_node)
if list_node
print "#{list_node.value} --> "
print_values(list_node.next_node)
else
print "nil\n"
return
end
end
class Stack
attr_reader :data
def initialize
#data = nil
end
def push(value)
#data = LinkedListNode.new(value, #data)
end
def pop
return nil if #data.nil?
returning_value = #data.value
#data = #data.next_element
returning_value
end
end
def reverse_list(list)
stack = Stack.new.push(list.value)
list = list.next_node
while list
stack.push(list.value)
list = list.next_node
end
stack.pop
end
node1 = LinkedListNode.new(37)
node2 = LinkedListNode.new(99, node1)
node3 = LinkedListNode.new(12, node2)
revlist = reverse_list(node3)
print_values(revlist)
# should return 37 --> 99 --> 12 --> nil
I get errors when calling the Stack class in the reverse_list method (undefined method push for <Context::LinkedListNode:0x00000001c5e0a8>).
I'm at a loss as to why I cannot use Stack, push, and pop inside reverse_list. Any tips as to how I could go about implementing reverse_list will be well received.

Answer 1

The following pushes the elements into the stack, and then reverses them:
def reverse_list(list)
stack = Stack.new
while list
stack.push(list.value)
list = list.next_node
end
LinkedListNode.new(stack.pop, stack.data)
end

Answer 2

stack = Stack.new.push(list.value)
In your example, variable stack is an instance of LinkedListNode as push method is returning instance of linked list. So no method error is throwing. You have to create instance of Stack first like stack = Stack.new which woulb be an empty stack and then do push and pop operation using this instance.

Answer 3

Your pop Method within your Stack class is calling for "next_element", but I think you should be referring to the "next_node" variable from the LinkedListNode class. I would re-write your "pop" method this way:
def pop
if #data.nil?
nil
else
returning_value = #data.value
#data = #data.next_node
returning_value
end
end
I would also rewrite the method to reverse the linked list like this:
def reversed_linked_list(list)
stack = Stack.new
while list
stack.push(list.value)
list = list.next_node
end
LinkedListNode.new(stack.pop, stack.data)
end

Answer 4

Iterate through the list adding its contents to a stack until the end of the list. After that, the following algorithm can be applied.
root = stack.pop() # last element
current = root
while not stack.empty(): # move in reverse direction
current.next = stack.pop() # next element in stack is the next node
current = current.next # continue to the next node
current.next = nil # mark end of the list