Python program to reverse a stack

The stack is a linear data structure which works on the LIFO concept. LIFO stands for last in first out. In the stack, the insertion and deletion are possible at one end the end is called the top of the stack.

In this article, we will see how to reverse a stack using Python.

Algorithm:

• Define some basic function of the stack like push(), pop(), show(), empty(), for basic operation like respectively append an item in stack, remove an item in stack, display the stack, check the given stack is empty or not.
• Define two recursive functions BottomInsertion() and Reverse()..

BottomInsertion() : this method append element at the bottom of the stack and  BottomInsertion accept two values as an argument first is stack and the second is elements, this is a recursive method.

# insert element at the bottom of the stack
def BottomInsert(s, value):
    # if stack is empty then call push() method.
    if s.empty():  
        s.push(value)
        
    # if stack is not empty then execute else
    # block
    else:
    
        # remove the element and store it to
        # popped  
        popped = s.pop()
        
        # invoke it self and pass stack and value 
        # as an argument.
        BottomInsert(s, value)
        
        # append popped item in the bottom of the stack 
        s.push(popped)

Reverse() : the method is reverse elements of the stack, this method accept stack as an argument Reverse() is also a Recursive() function. Reverse() is invoked BottomInsertion() method for completing the reverse operation on the stack.

# Reverse()
def Reverse(s): 

    # check the stack is empty of not  
    if s.empty():
    
        # if empty then do nothing
        pass
        
    # if stack is not empty then 
    else:
    
        # pop element and stare it to popped
        popped = s.pop()
        
        # call it self ans pass stack as an argument
        Reverse(s)
        
        # call BottomInsert() method and pass stack
        # and popped element as an argument
        BottomInsert(s, popped)

Below is the implementation.

Python3

# create class for stack
class Stack:

    # create empty list
    def __init__(self):
        self.Elements = []

    # push() for insert an element
    def push(self, value):
        self.Elements.append(value)

    # pop() for remove an element
    def pop(self):
        return self.Elements.pop()

    # empty() check the stack is empty of not
    def empty(self):
        return self.Elements == []

    # show() display stack
    def show(self):
        for value in reversed(self.Elements):
            print(value)

# Insert_Bottom() insert value at bottom
def BottomInsert(s, value):

    # check the stack is empty or not
    if s.empty():

        # if stack is empty then call
        # push() method.
        s.push(value)

    # if stack is not empty then execute
    # else block
    else:
        popped = s.pop()
        BottomInsert(s, value)
        s.push(popped)

# Reverse() reverse the stack
def Reverse(s):
    if s.empty():
        pass
    else:
        popped = s.pop()
        Reverse(s)
        BottomInsert(s, popped)


# create object of stack class
stk = Stack()

stk.push(1)
stk.push(2)
stk.push(3)
stk.push(4)
stk.push(5)

print("Original Stack")
stk.show()

print("\nStack after Reversing")
Reverse(stk)
stk.show()

Output:

Original Stack
5
4
3
2
1

Stack after Reversing
1
2
3
4
5

Time complexity: O(n) where n is the size of given stack

Auxiliary space: O(n)