#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <malloc.h>
struct stack
{
	int data;
	struct stack *next;
};

struct stack *top = NULL;
struct stack *push(struct stack *, int);
struct stack *display(struct stack *);
struct stack *pop(struct stack *);
int peek(struct stack *);

int main(int argc, char *argv[]) {
	int val, option;
	do
	{
		printf("\n *****MAIN MENU*****");
		printf("\n 1. PUSH");
		printf("\n 2. POP");
		printf("\n 3. PEEK");
		printf("\n 4. DISPLAY");
		printf("\n 5. EXIT");
		printf("\n Enter your option: ");
		scanf("%d", &option);
		switch(option)
		{
			case 1:
				printf("\n Enter the number to be pushed on stack: ");
				scanf("%d", &val);
				top = push(top, val);
				break;
			case 2:
				top = pop(top);
				break;
			case 3:
				val = peek(top);
				if (val != -1)
					printf("\n The value at the top of stack is: %d", val);
				else
					printf("\n STACK IS EMPTY");
				break;
			case 4:
				top = display(top);
				break;
		}
	}while(option != 5);
	return 0;
}

struct stack *push(struct stack *top, int val)
{
	struct stack *ptr;
	ptr = (struct stack*)malloc(sizeof(struct stack));
	ptr -> data = val;
	if(top == NULL)
	{
		ptr -> next = NULL;
		top = ptr;
	}
	else
	{
		ptr -> next = top;
		top = ptr;
	}
	return top;
}

struct stack *display(struct stack *top)
{
	struct stack *ptr;
	ptr = top;
	if(top == NULL)
		printf("\n STACK IS EMPTY");
	else
	{
		while(ptr != NULL)
		{
			printf("\n %d", ptr -> data);
			ptr = ptr -> next;
		}
	}
	return top;
}

struct stack *pop(struct stack *top)
{
	struct stack *ptr;
	ptr = top;
	if(top == NULL)
		printf("\n STACK UNDERFLOW");
	else
	{
		top = top -> next;
		printf("\n The value being deleted is: %d", ptr -> data);
		free(ptr);
	}
	return top;
}

int peek(struct stack *top)
{
	if(top==NULL)
		return -1;
	else
		return top ->data;
}