Problem

Given a list of integers, write a Python class that implements a stack using a linked list. The stack should have the following methods:
- push(x): Push an integer x onto the stack.
- pop(): Pop and return the top element of the stack. If the stack is empty, return None.
- is_empty(): Return true if the stack is empty, and false otherwise.
Example input:
push(1)
push(2)
push(3)
Example output:
3
2
1

Solution

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class Stack:
    def __init__(self):
        self.head = None

    def push(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    def pop(self):
        if self.head is None:
            return None
        else:
            data = self.head.data
            self.head = self.head.next
            return data

    def is_empty(self):
        return self.head is None

stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)

print(stack.pop())
print(stack.pop())
print(stack.pop())
print(stack.pop())