Stacks and Queue using Linked List data Structure

DS Practical-9 || Queue Using Linked List.c

#include <stdio.h>
#include <stdlib.h>
//todo structure & pointers declaration
struct node
{
    int data;
    struct node *next;
};
struct node *front, *rear, *newnode, *temp;
int flagforstart=0;//?this zero indicates that there is no element in queue at start
// todo function prototyping
void enqueue();
void dequeue();
void peek();
void display();

//todo enqueue function
void enqueue(){
    if (flagforstart == 0){
        front=NULL;
        newnode=(struct node*) malloc(sizeof(struct node));
        printf("\nEnter The Value you want to enqueue : ");
        scanf("%d",&newnode->data);
        newnode->next=NULL;
        front=rear=newnode;//!idhar teen log pehle wale pe point karenge
        flagforstart++;//!indicates one linked list is added
    }
    else
    {
        newnode = (struct node *)malloc(sizeof(struct node));
        printf("\nEnter The Value you want to enqueue : ");
        scanf("%d", &newnode->data);
        newnode->next =NULL;
        rear->next = newnode;
        rear = newnode;
        flagforstart++; //!indicates one linked list is added
    }

}

//todo enqueue function
void dequeue(){
    if(flagforstart==0){
        printf("\nQueue Underflow\n");
    }
    else
    {
        temp = front;
        front = temp->next;
        free(temp);
        flagforstart--;
    }
}
//todo peek function
void peek()
{
    temp = front;
    if (flagforstart == 0)
    {
        printf("\nEmpty stack\n");
    }
    else if (front == NULL)
    {
        printf("The element on Front of Queue is : %d\n", front->data);
    }
    else
    {
        printf("The element on Front of Queue is : %d\n", temp->data);
    }
}
//todo display function
void display()
{
    temp =front;
    if (flagforstart == 0)
    {
        printf("\nEmpty Queue\n");
    }
    else
    {
        printf("\nThe Queue is : \n");
        while (temp != NULL)
        {
            if (front==NULL)
            {
                printf("%d\n", front->data);
            }
            else
            {
                printf("%d\n", temp->data);
            }
            temp = temp->next;
        }
    }
}
//todo main function
int main()
{
    int option = 0;
    do
    {
        printf("\n***--------Main Menu--------***");
        printf("\nPress the following keys for following functions");
        printf("\n1) Enqueue\n2) Dequeue\n3) Peek\n4) Display\n5) Exit\n");
        scanf("%d", &option);
        switch (option)
        {
        case 1:
            enqueue();
            break;
        case 2:
            dequeue();
            break;
        case 3:
            peek();
            break;
        case 4:
            display();
            break;
        case 5:
            //so that default is not displayed when the option 5 is selected
            break;
        default:
            printf("\nInvalid Option key is pressed\n");
            break;
        }
    } while (option != 5);
    return 0;
}

DS Practical-9 || Stack Using Linked List.c

#include<stdio.h>
#include<stdlib.h>

//todo structure & pointers declaration
struct node {
    int data;
    struct node *next;
};
struct node *top, *newnode, *temp;
int flagforstart=0;
// todo function prototyping
void push();
void pop();
void peek();
void display();

//** here comes the functions 
//todo push function
void push() {
    if (flagforstart==0) {
        //for adding the first element to the stack
        top=NULL;
        newnode=(struct node*)malloc(sizeof(struct node));
        printf("\nEnter the Value you want to add to the stack: ");
        scanf("%d", &newnode->data);
        newnode->next=NULL;
        top=temp=newnode;
        flagforstart++;        //this indicates we created at least on node
    }
    else {
        //for adding the elements 
        newnode=(struct node*)malloc(sizeof(struct node));
        printf("\nEnter the Value you want to add to the stack: ");
        scanf("%d", &newnode->data);
        newnode->next=top;
        top=newnode;
        flagforstart++;
    }
}
//todo pop function
void pop() {
    if (flagforstart==0) {
        printf("\nStack Underflow\n");
    }
    else {
        temp=top;
        top=temp->next;
        free(temp);
        flagforstart--;
    }

}
//todo peek function
void peek() {
    temp=top;
    if (flagforstart==0) {
        printf("\nEmpty stack\n");
    }
    else if (top==NULL) {
        printf("The element on top of stack is : %d\n", top->data);
    }
    else {
        printf("The element on top of stack is : %d\n", temp->data);
    }
}
//todo display function
void display() {
    temp=top;
    if (flagforstart==0) {
        printf("\nEmpty stack\n");
    }
    else {
        printf("\nThe Stack is : \n");
        while (temp!=NULL) {
            if (top==NULL) {
                printf("%d\n", top->data);
            }
            else {
                printf("%d\n", temp->data);
            }
            temp=temp->next;
        }
    }
}

//todo main function
int main()
{

    int option=0;
    do {
        printf("\n***--------Main Menu--------***");
        printf("\nPress the following keys for following functions");
        printf("\n1) Push\n2) Pop\n3) Peek\n4) Display\n5) Exit\n");
        scanf("%d", &option);
        switch (option) {
        case 1: push();
            break;
        case 2:pop();
            break;
        case 3:peek();
            break;
        case 4:display();
            break;
        case 5:
            //so that default is not displayed when the option 5 is selected
            break;
        default: printf("\nInvalid Option key is pressed\n");
            break;
        }
    } while (option!=5);
    return 0;
}