Untitled

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

struct node {
    int data;
    struct node *next;
};

struct node *top = NULL;

void push(int value) {
    struct node *new_node = (struct node*) malloc(sizeof(struct node));
    new_node->data = value;
    new_node->next = top;
    top = new_node;
}

void pop() {
    if (top == NULL) {
        printf("Stack is empty.\n");
        return;
    }
    struct node *temp = top;
    top = top->next;
    free(temp);
}

int peek() {
    if (top == NULL) {
        printf("Stack is empty.\n");
        return -1;
    }
    return top->data;
}

int main() {
    int input, value;
    while (1) {
        scanf("%d", &input);
        if (input == -1) break;
        switch(input) {
            case 1:
                scanf("%d", &value);
                push(value);
                break;
            case 2:
                pop();
                break;
            case 3:
                printf("%d\n", peek());
                break;
            default:
                printf("Invalid input.\n");
        }
    }
    return 0;
}