Question

I’m doing some practise questions. This one needs to reverse a stack without using any other data structures except another stack.

I know I will need a helper function that appends the pop-ed numbers once the original stack is empty.

Can somebody get me started? I’m stuck here

def flip_stack(s):
    if not s.is_empty():
        temp = s.pop
        flip_stack(s)

Thanks!

The Stack class has pop, push and is_empty functions.

Answer 1

Here’s another possibility, using an accumulator and a helper function. I’m only using the methods provided in your Stack class, and no other data structures (such as Python’s lists) :

def flip_stack(s):
    return flip_stack_helper(s, Stack()) # Stack is your stack class

def flip_stack_helper(s, t):
    if s.is_empty():
        return t
    t.push(s.pop())
    return flip_stack_helper(s, t)

Be aware that the original stack will be empty at the end, and the flipped stack is returned.