## Introduction to Python Stacks

Have you ever wondered what a stack is or how to implement it in Python? A stack is a data structure that follows the Last In First Out (LIFO) principle, meaning that the last element added to the structure will be the first one removed.

Python stacks have several properties, including being unidirectional, having a top pointer, and being either dynamic or static.

One of the most common tasks when working with stacks in Python is to check if two stacks are equal.

In this article, we’ll explore two methods to check the equality of two stacks in Python. We’ll start by discussing the first method, which involves comparing and popping the top element of both stacks.

### Method 1: Compare and pop the top element of the two stacks

Before we dive into the implementation of the first method, let’s take a look at the algorithm behind it.

### Algorithm for Method 1:

- Compare the sizes of the two stacks. If they are not equal, return False.
- Create a loop that runs until the stacks are empty.
- Pop the top element of both stacks.
- Compare the popped elements.
- If they are not equal, return False.
- Continue the loop until the stacks are empty.
- If the loop completes without returning False, return True.

## Implementation in Python code

```
def equal_stacks(stack1, stack2):
# compare the sizes of the two stacks
if len(stack1) != len(stack2):
return False
# loop until the stacks are empty
while stack1 and stack2:
# pop the top element of both stacks
top1 = stack1.pop()
top2 = stack2.pop()
# compare the popped elements
if top1 != top2:
return False
# check if both stacks are empty
if not stack1 and not stack2:
return True
else:
return False
# driver code
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 3, 4, 5]
print(equal_stacks(stack1,stack2)) # Output: True
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 3, 4, 6]
print(equal_stacks(stack1,stack2)) # Output: False
```

We start by defining the `equal_stacks`

function that takes in two parameters: `stack1`

and `stack2`

.

In the first line of the function, we compare the sizes of both stacks. If they are not equal, we return `False`

.

Next, we enter the loop that runs until both stacks are empty. We pop the top element of both stacks and compare them.

If they are not equal, we return `False`

. If the loop completes without returning `False`

, we check if both the stacks are empty.

If they are, we return `True`

. Otherwise, we return `False`

.

In the driver code, we create two stacks and pass them to the `equal_stacks`

function. The first test case returns `True`

, while the second test case returns `False`

.

## Conclusion:

In this article, we explored the basics of Python stacks and their properties. We then discussed the first method of checking the equality of two stacks in Python, where we compared and popped the top element of both stacks.

We implemented the method using Python code and ran some test cases to check its validity. We hope this article has been informative and helpful in your journey to learning Python stacks.

## Introduction to Python Stacks

A stack is a fundamental data structure used in computer science to store and retrieve data. In Python, a stack follows the Last In First Out (LIFO) principle, meaning that the last element added to the stack will be the first one removed.

In this article, we will explore two methods to check equality of two stacks in Python and dive into the implementation details.

## Properties of Python Stacks

Before diving into the methods of checking stack equality, let’s briefly discuss the properties of a Python stack. Firstly, Python stacks are unidirectional, meaning that data can only be added or removed from the top of the stack, and not from any other direction.

Secondly, a Python stack has a top pointer that indicates the top element of the stack. When an element is pushed onto the stack, the pointer moves up, whereas when an element is popped from the stack, the pointer moves down.

Lastly, a Python stack can be either dynamic or static. A dynamic stack can grow or shrink in size during runtime, whereas a static stack is assigned a fixed size at the time of declaration.

### Method 1: Compare and Pop the Top Element of the Two Stacks

The first method of checking the equality of two stacks in Python involves comparing the top elements of both stacks and popping them if they are equal. The algorithmic steps involved in this method are:

### Algorithm for Method 1:

- Compare the sizes of both stacks. If they are not equal, return False.
- Create a loop that runs until both stacks are empty.
- Compare the top elements of both stacks.
- If the top elements are equal, pop both the elements.
- If the loop completes without returning False, return True.

Now, let’s implement the above algorithm in Python:

## Implementation in Python Code

```
def equal_stacks(stack1, stack2):
# compare the sizes of the stacks
if len(stack1) != len(stack2):
return False
while stack1 and stack2:
#compare the top element of both stacks
if stack1[-1] == stack2[-1]:
stack1.pop()
stack2.pop()
else:
return False
#check if both stacks are empty
if not stack1 and not stack2:
return True
else:
return False
#driver code
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 3, 4, 5]
print(equal_stacks(stack1, stack2)) #Output: True
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 5, 4, 3]
print(equal_stacks(stack1, stack2)) #Output: False
```

The `equal_stacks`

function takes two parameters, `stack1`

and `stack2`

, and compares the size of both stacks. If the sizes are not equal, the function returns `False`

.

If both stacks are of the same size, the function enters a loop that runs until both stacks are empty. It compares the top element of both stacks; if they are equal, the function pops both elements from the stacks.

If the top elements are not equal, the function returns `False`

. If both stacks are empty and the loop completes, the function returns `True`

.

### Method 2: Compare the Top Elements of the Two Stacks Without Alteration

Method two of checking the equality of two stacks in Python involves simply comparing the top elements of both stacks without popping any elements. This method is simpler than method one, but it requires more memory as it creates temporary stacks.

### The algorithmic steps involved in this method are:

### Algorithm for Method 2:

- Compare the sizes of both stacks.
- If they are not equal, return False.
- Create two empty stacks,
`temp1`

and`temp2`

. - Push the top elements of both input stacks onto their corresponding temporary stacks.
- Compare the top elements of both temporary stacks.
- If they are equal, pop the top elements from the input stacks.
- If the input stacks are empty and the temporary stacks are not empty, return False.
- Otherwise, repeat steps 3-6 until both input stacks are empty.
- Return True if the input stacks are empty and both temporary stacks are empty.

Now, let’s implement the above algorithm in Python:

## Implementation in Python Code

```
def equal_stacks(stack1, stack2):
# compare the sizes of the stacks
if len(stack1) != len(stack2):
return False
temp1, temp2 = [], []
while stack1 and stack2:
temp1.append(stack1.pop())
temp2.append(stack2.pop())
if temp1[-1] != temp2[-1]:
return False
if stack1 or stack2 or temp1 or temp2:
return False
else:
return True
#driver code
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 3, 4, 5]
print(equal_stacks(stack1, stack2)) #Output: True
stack1 = [1, 2, 3, 4, 5]
stack2 = [1, 2, 5, 4, 3]
print(equal_stacks(stack1, stack2)) #Output: False
```

The `equal_stacks`

function takes two parameters, `stack1`

and `stack2`

, and compares the size of both stacks. If the sizes are not equal, the function returns `False`

.

If both stacks are of the same size, the function creates two empty stacks, `temp1`

and `temp2`

. It then enters a loop that runs until both input stacks are empty.

The function pushes the top element of both input stacks onto their respective temporary stacks and compares them. If the top elements are not equal, the function returns `False`

.

If both input stacks are empty, the function checks if both temporary stacks are also empty. If they are, the function returns `True`

.

If not, the function returns `False`

.

## Conclusion

In this article, we explored two methods of checking the equality of two stacks in Python. The first method involves comparing and popping the top elements of both stacks until both stacks are empty.

The second method involves simply comparing the top elements of both stacks without popping any elements. Although the second method is simpler, it requires more memory.

We implemented both methods using Python code and ran test cases to check their validity. We hope you now have a better understanding of Python stacks, their properties, and how to check their equality.

In brief, this article covered two methods for checking the equality of two Python stacks: comparing and popping the top elements of each stack, and comparing the top elements of both stacks without altering them. We discussed the properties of Python stacks, implemented both methods using Python code, and provided test cases to validate their effectiveness.

The importance of understanding Python stacks and their equality is paramount for any software developer, and this article provides a solid framework for comprehending those concepts. In summary, with Python stacks playing a crucial role in programming, it’s essential to know how to check their equality when needed.