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Stacks / Constrains

Stacks are simply arrays with restrictions. We can thing at a stack as a list displayed vertically. Just like a stack of dishes, you can look only at the face of the top dish. A handy acronym to describe stack operations is LIFO (last in, first out).

Lists / Python

In Python the stack is build-in data type. It is called list and has append() and pop() methods.

 
# Empty list to represent a stack
mystack = []

# Push elements onto the stack
mystack.append(1)
mystack.append(2)
mystack.append(3)

# Output stack
print("Stack after insertion =", mystack)

# Pop element from the stack
front = mystack.pop()

# Output stack
print("Stack after deletion =", mystack)
print("Popped element, front =", front)

"""
    Stack after insertion = [1, 2, 3]
    Stack after deletion = [1, 2]
    Popped element, front = 3
"""

Create Stack

Most programming languages doesn't come with a built-in stack data type. Let's create a stack to see how it works internaly.

 
class Stack:
    def __init__(self):
        self.items = []

    def push(self, item):
        self.items += [item]

    def pop(self):
        item = self.items[-1]
        self.items = self.items[:-1] # remove the last from the list
        return item

    def read(self):
        return self.items[-1]

# New stack
stack = Stack()

# Push elements onto the stack
stack.push(1)
stack.push(2)
stack.push(3)

# Read from stack
print("Read from stack top =", stack.read())

# Pop from stack
popped = stack.pop()
print("Remove from stack top =", 

"""
    Read from stack top = 3
    Remove from stack top = 3
    New top = 2
"""