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Lecture 1 (Learning)

Implment Stack using arrays

create a stack class that has

• push() : insert the element on top of stack
• pop() : remove the element from the top
• top() : return the top most element
• size(): return the total number of elements in the stack

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

    def push(self,element):
        self.elements.append(element)

    def pop(self):
        if not self.elements:
            return None
        
        return self.elements.pop()

    def top(self):
        if not self.elements:
            return None
        
        return self.elements[-1]

    def size(self):
        return len(self.elements)

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Implement Queue using array

Example:

Input: push(4) push(14) push(24) push(34) top() size() pop()

Output:

The element pushed is 4 The element pushed is 14 The element pushed is 24 The element pushed is 34 The peek of the queue before deleting any element 4 The size of the queue before deletion 4 The first element to be deleted 4 The peek of the queue after deleting an element 14 The size of the queue after deleting an element 3

class Queue:
    def __init__(self):
        self.elements = []
        self.front_idx =0

    def is_empty(self):
        return self.front_idx >= len(self.elements)

    def push(self,element):
        self.elements.append(element)

    def pop(self):
        if self.is_empty():
            return None

        val = self.elements[self.front_idx]
        self.front_idx +=1
        return val

    def peek(self):
        if self.size()==0:
            return None

        return self.elements[self.front_idx]

    def size(self):
        return len(self.elements)

The element pushed is  4
The element pushed is  14
The element pushed is  24
The element pushed is  34
The peek of the queue before deleting any element  4
The size of the queue before deletion  4
The first element to be deleted 4
The size of the queue before deletion  3

Approach2 (Built in)

from collections import deque

class Queue:
    def __init__(self):
        self.elements = deque()

    def is_empty(self):
        return len(self.elements) ==0 

    def push(self,element):
        self.elements.append(element)

    def pop(self):
        if self.is_empty():
            return None

        return self.elements.popleft()

    def peek(self):
        if self.is_empty():
            return None

        return self.elements[0]

    def size(self):
        return len(self.elements)

Implment stack using linked list

Following operations can be performed in the stack:

push(): pushes an element at the top of the stack. pop(): removes an element from the top of the stack. size(): returns the size of the stack. isEmpty(): returns a boolean value for whether the stack is empty or not. peek()/top(): returns the top element of the stack.

class Node:
    def __init__(self,val=0,next=None):
        self.val = val
        self.next = next

class Stack:
    def __init__(self):
        self.head = None
        self.size =0

    def push(self,element):
        new_node = Node(element)
        new_node.next = self.head
        self.head = new_node
        self.size +=1
        

    def pop(self):
        if self.isEmpty():
            return None

        temp = self.head
        self.head = self.head.next
        val = temp.val
        del temp
        self.size -=1
        return val

    def size(self):
        return self.size

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

    def peek(self):
        if self.isEmpty():
            return None

        return self.head.val