Linked Lists & Stacks

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

// Singly linked list
typedef struct Node {
    int data;
    struct Node *next;
} Node;

Node *createNode(int data) {
    Node *node = (Node *)malloc(sizeof(Node));
    node->data = data;
    node->next = NULL;
    return node;
}

void insertFront(Node **head, int data) {
    Node *newNode = createNode(data);
    newNode->next = *head;
    *head = newNode;
}

void printList(Node *head) {
    while (head != NULL) {
        printf("%d -> ", head->data);
        head = head->next;
    }
    printf("NULL\n");
}

void freeList(Node *head) {
    while (head) {
        Node *temp = head;
        head = head->next;
        free(temp);
    }
}

// Stack (array-based)
typedef struct {
    int *data;
    int top;
    int capacity;
} Stack;

Stack *createStack(int capacity) {
    Stack *s = (Stack *)malloc(sizeof(Stack));
    s->data = (int *)malloc(capacity * sizeof(int));
    s->top = -1;
    s->capacity = capacity;
    return s;
}

void push(Stack *s, int value) {
    if (s->top >= s->capacity - 1) return; // Overflow
    s->data[++(s->top)] = value;
}

int pop(Stack *s) {
    if (s->top < 0) return -1; // Underflow
    return s->data[(s->top)--];
}

int main() {
    // Linked list usage
    Node *list = NULL;
    insertFront(&list, 30);
    insertFront(&list, 20);
    insertFront(&list, 10);
    printList(list); // 10 -> 20 -> 30 -> NULL
    freeList(list);

    // Stack usage
    Stack *stack = createStack(10);
    push(stack, 1); push(stack, 2); push(stack, 3);
    printf("Pop: %d\n", pop(stack)); // 3 (LIFO)
    free(stack->data); free(stack);

    return 0;
}